Senator Levin in Michigan is probably going to retire after the current term. Michigan has become a solid Blue state in the last 10 years so even a pickup here with no incumbent is unlikely.
GOP Incumbents up in 2008:
Wayne Allard - CO (very vulnerable)
purple swing state, will take lots of $$ better here than NJ or Me
Susan Collins - ME (likely safe)
Thad Cochran - MS (likely safe)
Elizabeth Dole - NC (possibly vulnerable)
Pete Domenici - NM (likely retirement)Another purple state for pres EV's
Chuck Hagel - NE (likely safe)
James Inhoffe - OK (GOP advantage)
Pat Roberts - KS (vulnerable, if Dems recruit Sebelius)Safe if not
Ted Stevens - AK (safe, as long as he lives)Possible retirement
John Warner - VA (very vulnerable, Mark Warner will run)
Lamar Alexander - TN likely safe
Saxby Chambliss - GA (likely safe)
Norm Colemans - MN (very vulnerable)Presidential EV's At stake too
John Cornyn - TX (likely safe)
Lindsey Graham - SC (safe Republican)Needs to be primaried
Mitch McConnell - KY (GOP advantage)
Jeff Sessions - AL (likely safe)
Gordon Smith - OR (vulnerable -- it's Oregon)
John Sununu -- NH (very vulnerable -- NH trending badly)Spend $$$ get our house seats back too and Get EV's for pres
Mike Enzi -- WY (likely safe)
Go after "Red Earth" Johnson, Landreiu and Baucus
We have too much to defend to go after Pryor, Harkin or Lautinbergs seats.