This may be an old thread but I wanted to add my experience as well. I”m running WP 2.7.1 and got the same thing – tried to log in and got a blank page.
After reading a few of the posts here, I FTP’d to my site and went to my plugins folder…I saw ONE plugin folder that was dated yesterday’s date, so I just changed the name of the folder by adding “zzz” to the end.
After doing this I was able to log in.
I’m really not sure what happened.. I’m just glad to be back in.
FYI – the plugin was WP-Polls which is an Ajax poolling plugin.
(https://lesterchan.net/wordpress/readme/wp-polls.html)
PLEASE NOTE: I’m not crapping on this guy’s plugin… it’s worked perfectly until now… Don’t know what happened