Encountered the same issue after upgrading to WP 2.5 tonight.
I was still able to access the admin interface (via “edit this entry” of a random blog post). Went to “plugins” and clicked “deactivate all”. Even though all plugins were already listed as inactive (which I had double-checked before, according to instructions), that apparently fixed it for me.
My current plugins:
Akismet 2.1.4
Disable WordPress Widgets 1.0
flickrRSS 3.2
Hello Dolly 1.5
wp-Table 1.51
Hope this helps…
