Where are Address Book group definitions stored and how can I delete them all?
I think I can answer my own question, based on further experiments.

Once you sync an Address Book with iCloud, the data is, in a sense "gone" from the local drive. You can still see it all in the usual location, but it's a cached version of the iCloud data. (You can "move" contacts back to the local drive, but Groups are gone forever.)

On startup, even if you're not logged into iCloud, there's a process (a bug?) that recreates at least the Groups, probably from some local iCloud data store. (I do wonder if Apple has 'social' plans four our Groups.)

If you truly kill all iCloud accounts, including from both System Preferences AND from Address Book, and restart, it is possible to delete all from Address Book and not have files magically reappear.

There are many tedious details on this and other iCloud topics in a long personal multi-update blog post: http://tech.kateva.org/2012/06/icloud-transition-went-as-expected.html.

Suffice to say, these are coming from iCloud. If you never mess with iCloud (personally I'm going to wait for Mountain Lion!) you won't run into this bizarre behavior.