Senate Races 2006.

[Sam Spade]

Bob Casey Jr. would probably stand the best chance.  Cuttin into Santorum's western PA appeal is about the only way to beat him.

It the other parties can get good challengers, these races might be interesting:

Dems
Debbie Stabenow, MI (she only won 49% last time)
Hillary Clinton, NY (only if Rudy runs)
Maria Cantwell, WA (Nethercutt will probably try again)
Jon Corzine, NJ (there's a good chance he'll run for Gov., would try to appoint his own replacement but NJ situation sort of unstable right now)
Paul Sarbanes, MD (if he retires, Michael Steele will definitely run)
Kent Conrad, ND (if the GOP can find another John Thune)
Ben Nelson, NE (he is very popular, but don't be surprised if Mike Johanns, a very pop. Gov runs against him)
You already mentioned Bill Nelson, Mark Dayton and Robert Byrd (if he retires, I agree on that too)

Reps
George Allen, VA (only if Mark Warner runs)
Jim Talent, MO (Dems will have to find a good candidate)
Rick Santorum, PA (it is unknown how much the Specter support cost him in his western PA base, still pretty tough actually, Dems have good candidates, Rendell machine)
Conrad Burns, MT (only if Schweitzer runs again)
Olympia Snowe, ME (only chance is in-party challenge)
Trent Lott, MS (if he retires)
Richard Lugar, IN (same here, Dems would have to find another Evan Bayh figure)
Bill Frist, TN (if he retires, Harold Ford would be formidable)

Objections:
Democrats are officially on life support in Texas, even if Hutchinson were to run for Gov., it would be pretty impossible for Dems to win, DeLay is wiping them out there as we speak.

No one will challenge Jim Jeffords, unless he leaves, then I give the Dems the edge on that one.

[Sam Spade]

I agree, though I might say that I don't think Warner can defeat Allen head-on (Allen is very powerful and very well-liked).  He'd probably end up with about the same percentage he did against Warner Sr. in 1996.  It might be smarter to just wait and see if Warner Sr. would retire in 2008.

Santorum is sometimes vulnerable, especially since he stupidly back Specter against Toomey, but I really don't know how that hurt him among his base, if it did at all.  If it didn't, he's a tough nut to crack in PA because of his strength in Western PA.  He always appears more in danger than he is (very few Senators like that).

Ford can win in Tennessee if Bill Frist retires.  He's conservative socially and that would play well there.  I don't know who would run against him on the Rep side.  He won't challenge if its against Frist, obviously.