I also made sure to install all available updates to the my Filemaker, and I have had a logout since, but it seems to be much less frequent. I think this supports my idea that the autosave is in conflict with other system operations, i.e. b/c it's writing much less frequently, there's less of a chance of conflict. I've also seen weird behavior if the file is in my Dropbox. In case you don't know what that it is, Dropbox automatically syncs a local folder to a remote web directory (a great and free service). When the file is in the local Dropbox folder, I see via Growl notifications that it is saving out repeatedly and creating conflicting versions.
I am also using Filemaker Pro Advanced 8.0, but I doubt there's enough difference b/w 7.0 and 8.0 to make a difference. You might try setting to 30 minutes or longer and see if that helps.
As for what precipitates the logout, tabbing between fields will cause the cache to update so it may still be this caching problem. Of course, it's really a Snow Leopard problem that Apple needs to fix. I know they want to shed legacy software support so they don't have a great incentive to make Rosetta work well, but we paid for this freakin' update and they need to take care of us.
Anyone know if they are indexing the drive differently under Snow Leopard? It also seems that the heretofore useless Spotlight works much better, so maybe the indexing scheme is different and is involved in causing the problem.