Feingold must have decided that he doesn't want to be the moveon.org 2008 candidate.
I'm sure Feingold wants to create a (phony) image of moderation and that this played into his 'yes' vote, but as with Leahy and the others its probably more so in preparation for the chance that Bush actually nominates a much more solidly known conseravtive for O'Connor's seat. Then they can attck that candidate without mercy and then claim that 'if only Bush had nominated another mainstream conservative (i.e. another O'Connor type they hope) like Roberts' then they wouldn't have to come out so forcefully against the racist, sexist, homophobic, and regressive radical conservative nominee.