Gmail definitely allows +omnifocus, but I did a _lot_ of testing this week (so far) regarding Mail activating/not activating the applescript attached to the mail rule, and it doesn't always seem to work. When I send from Gmail's web interface to
derek+omnifocus@gmail.com (not real address, of course), it works! When I send from Mail.app to
derek+omnifocus@gmail.com, it doesn't.
The applescript doesn't even get called, so it looks like it might be a problem isolated entirely within Mail.
Work in progress..