Look at this last election. No incumbent fell close to the wire except for Daschle and Bunning. Both special cases (for different reasons, obviously).
There isn't a single Democrat or Republican up-in-2006 Senate incumbent who is truly vulnerable for the general election.
Cantwell, Stabenow, and Clinton are at most somewhat vulnerable, as is Santorum.
Chafee is technically vulnerable to lose the Republican primary -- but he's going to change parties, and cruise to re-election as a Democrat, if a serious opponent gets into the Republican primary. He'll switch earlier if Specter is denied the Judiciary chairmanship.
I agree with your analysis: virtually none of the 2006 incumbents look vulnerable, but perhaps a couple won't run, maybe Dayton in MN, and of course Hutchison in TX may seek the governorship. I am sure that Robert C. Byrd and EMK will continue to run. If Hutchison opposes Rick Perry in the gubernatorial primary, Lt. Gov. David Dewhurst would be the favorite to win the Senate seat.