The retirements are coming one after another, with Jeff Bingaman, Daniel Akaka, and now John Ensign retiring. I've talked about the likelihood of Republicans taking over the Senate in 2012 and almost every retirement has helped the GOP.
Without the retirements Republicans had virtually no chance in New Mexico or Hawaii, while the retirements in Nevada, North Dakota, and Virginia have made Republican pick-ups more likely. I don't think Republicans are favorites New Mexico or Hawaii, but I think they now have a 45% chance to win one or the other. It was 10% before the retirements. Retention is far more likely in Nevada now that the scandal plagued Ensign won't be running for re-election. Heather Wilson is considered the strongest GOP candidate in New Mexico and she's in. Republicans are hoping for Linda Lingle in Hawaii and if Dean Heller runs in Nevada, he's not a lock, but he will be a heavy favorite.
I'm putting the senate at 52.6-47.4 Republican right now and giving Republicans around a a 2/3 chance of winning the senate.