OK -- it was a slow day at work, so I put together a quick page of Republicans who have endorsed Kerry, as well as those who have publicly stated that they can't vote for Bush.
I have no pride of authorship, nor do I profess to know much about who's a prominent Republican, so please go and edit/add as necessary!
http://www.dkosopedia.com/index.php/Republicans_for_Kerry_2004