Good idea about trying activation with an AT&T SIM; the only catch might be the activation via SMS which could tie the FaceTime account on the iPhone to the telephone number on the SIM, it may not work once you put a non-AT&T SIM in the phone as the phone number may not match up with the device any more. You would have to go into Settings -> Phone -> My Number, and make sure your non-AT&T number is in there before trying the activation. I don't have very high hopes of it working though, but you never know.
If anyone's tried it, please post your experience with it here. The only AT&T iPhone micro-SIM I can get usually my hands on is out of the country right now, so I can't try it until next week.
In the meantime I have discovered that most everyone I know who has a FaceTime capable device is actually using Skype instead, which works pretty well but does not have the seamless integration with the device.
The assertion that any particular network is not a supported network does not seem to be borne out by Apple's documentation or lack thereof - they have yet to post a list of which providers are "supported". If this changes, please post a link to it. Rumors, or "facts" that "everyone knows" that aren't supported by documentation are not useful.
The repeated negative postings about networks or other things not being officially supported are not adding anything to the discussion and appear to be misguided attempts at flame postings which are prohibited here. It is not the place of any community member to pass judgement on others, and there is no reason to be making angy-sounding postings. There is no harm in others exploring the technology of their devices and trying to use it in different ways. If you can't participate in this discussion in a civil manner, it might be better to take a step back and just ignore it.
I'm not sure why there's such a heated discussion over this. It's just FaceTime, not a religious war.