I would not consider myself an expert, so feel free to correct me, but based on my understanding of the technology and my experience with numerous international/unlocked/non-branded (or whatever you want to call it) cell phones and many different carriers (both US and international), I'd like to share my thoughts on the topic.
I do believe that a non-US (AT&T) iPhone will function on a US GSM network, including T-Mobile USA. This most likely applies to any GSM iPhone working on any GSM carrier, given the appropriate operating frequencies (this is where quad-band comes into play). This is attributed to the fact that GSM is an international standard that is implemented by the manufacturers and carriers, regardless of location.
However...
Currently, our iPhones require iTunes activation. I am with minivini with regard to questioning the activation procedures of a non-US/AT&T iPhone. Will iTunes activation somehow prevent a regionalized iPhone from being activated in a region (country) that it was not sold in? Will the non-US SIM cards be 'married' to the iPhone, like the AT&T SIM? There are hacks available to bypass these for the US phone (e.g. jailbreak or unlocking) and a similar version of these will probably be developed for non-US phones, but until it comes out oustide of the US and groups like the iPhoneDevTeam are working on it, we have no way of determining this.
Purchasing an international phone usually involves some kind of risk in terms of difficulty of getting support of making warranty claims, regardless if it is a T-Mobile, Orange, Rogers, or whatever. I have seen this happen on Motorolas, Nokias, and Sony Ericssons, where I resorted to relying on forums like this and repairing the phone myself because either I had inadvertently waived my rights to a warranty through de-branding or unlocking, or the warranty required me to submit the claim through the carrier the phone was purchased from or to the service center which is tied to a particular locality. For example, prior to the iPhone, I had a Motorola ROKR E6. (note, this is not the ROKR that was released in the US with iTunes, but a new linux phone released only in Asia). A screen digitizer issue would have required me to ship the phone back to Asia to have it serviced, had I not fixed it myself using parts purchased via eBay. This might not be an issue for some. It would be for me, given the cost of replacement parts and the fact that I have already had my iPhone replaced due to a dropped incident. Lets say I had purchased my iPhone from T-Mo Germany and dropped it. Would Apple have so willingly and quickly replaced it?
My last thought is tied to functionality. As stated at the beginning of my post, I do not believe a non-US iPhone should have any trouble working on another network, including T-Mo USA. However, I have to question whether or not it will be fully functional. I am not talking making calls or receiving text messages or even visual voicemail or anything like that (as it has already been stated that a carrier needs to have the appropriate hardware for this to work). What I question is the data side of the phone. Although the GSM standard will allow phones to 'work' anywhere it is implemented, each carrier/locality may have different ways of handling the data side of things. I had heard that AT&T may utilize proxies to 'speed up' the edge experience for their subscribers. Will each carrier have similar proxies set up and will regionalized phones only be able to access their appropriate proxies? If these international iPhones are in fact 3G (as some of the rumors and leaks have indicated), this may be another data stubmling block. Carrier A in the US may provide 3G data through frequencies 1 and 2, however carrier B in EU may do so using frequencies 3 and 4. I have seen this on some Nokia N-series phones that I have owned. Although they were 3G and worked as so in EU, I was not able to take advantage of the AT&T 3G network. I also know that T-Mo USA's 3G network will supposedly be partially on a non-standard 3G frequency, which will effectivlely require T-Mo USA specific 3G phones. Now, there may be a hack or mod will be able to overcome any of these, but for now this is another potential issue that people should be aware of.
Alright, even though I know I could keep going, I'm gonna stop because this post is already longer than I had planned. Please do not dismiss my low post count as me being a noob or anything. I do not post unless I have a direct question that has not previously been answered or could provide a significant contribution to the discussion. In this case, I just hope that sharing these thoughts would provke some insightful discussion.