I have done a little research.  In the past 50 years (25 general elections), there have been 146 Senate retirements.  That is an average of over four retirements per election.  Last year, there were three retirements and one Senator facing the voters for the first time (New Jersey).  The fewest retirements in the Senate is 2 in 1964 and the most is 13 in 1996.  

It is a near certainty that someone up for election this year will decide to retire.  It is more likely that there will be several retirements than a few (the highest number of retirements came after the Republican seized both houses in an off-year election).  Many of the Senators are aging, many Republicans will be facing the prospect of being in the minority for a long time (at least 6 years).  There are a lot of Senators in the fourth quarter-century of their lives, who just aren't what they once were.  It makes little sense for a sitting Senator to announce his or her retirement so far away from the next election day.  

Also, men and women in their 70s and 80s don't have the greatest life expectancy.  21 Senators have died in office in the last half century, an average of almost one per election cycle.  I know of at least two who are battling cancer (not counting McCain's skin cancers).  The composition of the US Senate will probably be different for one reason or another by the time the next election rolls around.  It is very possible that without another ballot being cast that the Democrats lose their majority or maybe widen it.

This is an "off the top of my head" look at each of the fifty states, mostly looking at how possible it is for a Democratic presidential candidate to carry the state:

REPUBLICAN BASE:

These states haven't gone Democrat since I was old enough to vote (1972) and have a preponderance of Republican representing them and sitting in the Governor's chair.

1)  Idaho:  LBJ carried Idaho in '64.  No Democrat has been close since.  Both Congressmen and both Senators are Republicans, the Governor-elect is Republican.  I haven't checked, but I am pretty sure the legislatures in the state are solidly red.  It has been a long time since Frank Church and Cecil Andrus.  

2)  Mississippi:  In the solid red category, I am going to switch back and forth between the deep South and the inner-Mountain West.  Mississippi has two Democratic representatives, but clearly the national Democratic party is not looked on with favor.  Jimmy Carter carried Mississippi in '76, but another "son of the South", Bill Clinton, couldn't carry the state in his two successful elections.  

3)  Utah:  While the state has one Democratic representative, the numbers look daunting for any Democrat to carry the state.  Even W is popular here.  Two Republican US Senators.  LBJ carried Utah in '64, it hasn't really been close since.

Well, the Democrats have at 232 seats with a runoff in TX-23, which could give them more seats than the Republicans before this election.  There also are five races left on the board, all with the Republicans leading, which could add to the Democrats advantage.

Here is my handicap of the remaining races:

OH-02  <1%--Provisional votes outstanding don't favor Wulsin enough to make up for the deficit.  We will have Mean Jean to kick around for two more years.  BTW, I read the story on Schmidt in Rollingstone.  It is pretty strong.

NM-01 2%--Not enough allowed provisionals and "in-lieu-of" votes remain for Madrid to pull it out.  The only way Madrid wins is if a) the margin is close enough for there to be a recount and a major (1000 vote error) is found.  b) Madrid wins a challenge and gets most or all of the disallowed votes counted.

FL-13 20%--IMHO, Jennings won this race, but won't be able to prove it.  The recount won't win back the undervote, which should have gone overwhelmingly to Jennings.  She will lose the recount.  Legal action might force a new election, which would favor Jennings because of the appearance of trying to steal an election plus the fact that Jennings would be in the majority.

In the order of possibility of Democrats getting the seat, here are my ratings of the House elections still undecided:

LA-02:  100%. Easy one.  Will go to a Democrat.  I presume it will be Carter, but wouldn't be stunned if Jefferson is re-elected.

CT-02:  75%.  With all votes in, Courtney leads by 167 votes.  I believe Courtney will prevail.  The race will be recounted.

GA-12:  75%.  889 votes separate the candidates.  Some votes to be counted.  Again, a likely recount.  I trust Georgia less than Connecticut to be accurate, but Barrow has a larger lead.

OH-15:  20%.  There are plenty of provisional ballots that could lift Kilroy over Pryce.  I think it will be less than 1000 votes, but Pryce prevails.

TX-23:  20%.  If Republican fatigue sets in with the voters there, Rodriquez could squeak in.  Bonilla has lost his power since his party lost the House.

NC-08:  15%. Again, there are enough provisional votes to swing an extremely close race.  There will probably be a recount and making up 456 votes is possible in NC.

Darcy Burner is trailing by about 3600 votes with 75% of the vote counted.  She appears to be headed for a narrow loss.  Burner would have to pick up 34,000 out of the remaining 63,000 votes.  This would be an increase to about 54% for the remaining votes to eke out a narrow win.

Washington woman Democrats have a history of doing just that.

The Democrats performance on Election Day was astounding when all factors are put in perspective.  In the House, they picked up roughly 30 seats despite substantial Gerrymanders, less money to spend and a relatively small number of retirements.  It has proven to be very hard to defeat an incumbent in the last 10 years even when people are unhappy with the direction of the country.  In the Senate, they defeated six incumbents to win the majority.  They had to defend two open seats while the Republicans had only one.  Six governorships were picked up.  Through all of this, not one Democratic seat was lost, even open ones.  

Credit has to go to the Democratic strategists.  Blame has to be placed on the previous majority party and their leader.  The most brilliant strategy wouldn't have netted half of the acquired seats if Bush had been a good president, with a job approval over 50%.  The Republican Congress is guilty of overreaching and being corrupt at the same time.  

Beyond that, the defeat could have been worse.  At least a dozen Republicans won house seats by two percent or less or with less than 50% of the vote.  Some of these "winners" would have lost if they had faced better candidates.  Others would have lost if their opponent was better-funded.    

I have stayed up all night watching Tester's lead slip.  I did my best extrapolating from precincts and counties and determined that Tester would win or lose by less than a thousand votes.  He has rallied in Yellowstone County to where it looks like he will triumph by right around one thousand.  I am sure the GOP will want and get a recount there and also in Virginia.  It warms my cold, black heart to see "my side" win such a close election, if it happens that way.  I have seen strange things happen--Gregoire and Cantwell winning days after the election in Washington (Gregoire in a recount), Tim Johnson getting enough Native American votes to win re-election to his Senate seat in '02--but it doesn't make up for the last two presidential elections.  I am sure the plurality of voters in Florida wanted Gore, and I can be convinced that the plurality of voters in Ohio wanted Kerry.  

I am not totally overjoyed by the election results.  It looks like my state (Minnesota) will re-elect a conservative Republican by a very narrow margin.  Patty Wetterling lost, and probably will not run again.  However, there is a new Congressman in District 1, a monumental upset.  

Looking at the lay of the land, there will be many recounts.  Virginia's Senate leading the way.  Also, FL-13, at least one race in Connecticut, and one in Pennsylvania.  Also, it looks like my hopes for a pure shutout are not going to happen.  One of the two Georgia seats looks like it will change hands.  Madrid and Wilson in the desert are separated by less than a thousand votes as I write this.  Mean Jean looks like she has won a squeaker in Ohio.  The Democrats will come in with something like 230 seats, an ample majority.  They will have to play defense on many of those seats in two years and may lose a few.