My understanding, from a contact within Apple, is that the reviewers are indeed Apple FTEs. It wasn't clear if they were somewhere on the main campus or in one of the satellite buildings, but I was led to believe they are in Cupertino.

Certainly some of the stats I was able to get from testers (thanks PinchMedia) suggest the testers have access to the AT&T network in the US. I could probably find IP addresses from Apple's /8 if people wanted to try to validate further.