Here's a few..
(note, most of these are rumors.. contribute more rumors if you want, yes, I know some of these districts are safe)
AZ-5 (Hayworth)
AZ-6 (Flake)
Hayworth is rumored to be running against Napolitano.
Flake pledged to serve 3 terms in 2000, he'll keep that pledge (apparently) and run against Napolitano.
AR-4 (Ross)
Possible run to succeed Huckabee.
CA-49 (Issa)
for some reason, politics1 thinks he could run against Ahnuld, but I doubt it.
FL-9 (Bilirakis) - retirement
FL-11 (Davis) - possible run for Governor
FL-6 (Stearns), FL-13 (Harris), FL-14 (Mack), FL-15 (Weldon), FL-16 (Foley) - all possibly running against Nelson. Expect at least 3 of them to not run.
ID-1 (Otter) - run for Governor
IA-1 (Nussle) - running for Governor (Vilsack is apparently retiring))
MN-6 (Kennedy) - he'll probably run for the Senate
MS-3 (Pickering) - maybe they didn't get the memo, but he's listed as a candidate for the Senate, probably if Lott changes his mind and retires.
NE-3 (Osborne) - a possible run for Governor
NV-2 (Gibbons) - possibly running for Governor
NY-3 (King) - possible run for Governor
NY-11 (Owens) - He's retiring
NY-20 (Sweeney) - either a run for Governor or Senator
I've heard retirement rumors about Bartlett (of Maryland), Ralph Hall, Regula, Bill Young and others.
any other rumors you want to mention?