OK, just started in on the Google thread, and think I see why this isn't working. Not sure I'm going to be able to get it to work, not in the security environment I work in...

Sounds like whatever user I choose to use for the "EXECUTE AS" needs to be a DB login as well, for each DB. My sysadmin account doesn't exist as a DB login, because hey, it's SA!

Ditto for the other account I've been trying to use.