You really don't have to do anything to the database at all.
You just have to follow the upgrade (manual instructions) steps 7-8, but use the old 3.0.5 zip file instead.
Instructions are here: http://codex.wordpress.org/Upgrading_WordPress_Extended
Old versions are here:
Basically, delete your wordpress files (except for the ones outlined by the Updating WordPress Extended instuctions (link above).
Then, upload version 3.0.5 and extract it. Visit your site and login, continue with the "database upgrade" even though it's a downgrade.