Ended up buying an unlocked 4S last night. The trick appears to be the terminology at this point. If you ask for an AT&T phone off contract, they hand you the phone and never scan it in their system. The first time it's attached to iTunes, it confirms it's unlocked. There is a pretty lengthy thread I found talking about this, with confirmation from people putting in various non AT&T SIMs and finding they worked fine. This includes T-Mobile, but as in the past, the phone drops to edge only. If something changes, I still have a 30 day no questions asked return period.

Now to figure out if the CDMA piece is indeed completely shut down. Though for my possible transition to prepaid service or to a different provider, I'll probably look to stick with one using AT&T's network. For data, their speeds still exceed what most people are seeing with the 4S on Verizon or Sprint by a noticeable amount. My primary use is data, and the motivation to switch payment/providers is to drop my mostly unused voice part of the bill.