Hey man…
I had the same problem with a friends site. You couldn’t login without getting a white screen. I tried turning error reporting on… and such and nothing.
Anyways… their problem was that favatars (sp?) plugin was outdated. Although it worked previously.. for whatever reason it didn’t work with the style/theme.
This was my 1st time trouble shooting a word press… but I checked out the site… updated the favatar via ftp (file replace) and the site acted like nothing was ever wrong!
Thanks for this post… it really helped trouble shooting!
TRY RE-INSTALLING/UPDATING YOUR PLUGINS!