Great plugin, it solves one of the missing pieces in our Google Apps login project.
With that said, it seems that while users can logout successfully, they can also log back in without being logged in to Google Apps.
Say I login to my MediaWiki install (whether or not I'm logged into Apps already) and I end up back at Main_Page. If I click log out, I'm logged out and everything is peachy keen. Now, if I'm also logged out of Apps at this point, I can log right back in just by clicking the log in link in the header. That is to say, I don't need to be logged into Apps again to actually authenticate to the wiki; it lets me right back in. I'm not sure what's going on here, I suspect it has to do with the temp files, but erasing them isn't solving the issue. On the off chance it was OS or browser specific, several have been tried (Safari, Chrome, Firefox, on Windows and OS X - all latest versions as of this writing). The (Apache on Debian) web server user has full r/w permissions to the tmp directory and subdirectories.
If you have any insight as to how to solve this "endlessly" persistent login problem, that would be greatly appreciated. We've come up empty handed thus far.