Then use any number of solutions to unlock it, and use it on whichever network you like.
As other people have pointed out, it's GSM, and while I do have t-mobile, the wireless internet service supported by iPhone is 3G, which sucks monkey balls as well. If I get an iPhone, it'll be some time down the line.
First I just want to clarify some jargon. GSM can mean either the group of technologies (including UMTS), or specifically the 2G network (the predecessor to UMTS).
Is it UMTS, or one of the other 50 million GSM 3g technologies? It doesn't even have a GSM fallback at all? I hear there are tons of deadspots, but I've never used it. In the CDMA world, you can use the CDMA2000 protocol over 1xRTT and it's considered 2.5G, because you get all of the features but none of the broadband.
My Verizon phone has 1xRTT (2g) and EV-DO (3g), and it's pretty rare that I don't have a signal on both. In the year that I've had my phone, I've only had one call dropped, and it's because I was moving from an area of only EV-DO coverage to an area of only 1xRTT coverage, for which there's no mechanism for switchoff.
[edit] Additionally, it has an 802.11 card in it; both CDMA and GSM providers are moving toward a [tradesecret] goal of enabling all ATs to be only 802.11 enabled, so they can VPN to the network and do VoIP calls similar to the way 3g networks work. Cell towers would broadcast a sort of 802.11 tunnel. [/tradesecret]