You can use whatever phone you want with those. All you need is to find a GSM phone that can run Android and your set. I don't know if any are out yet, I doubt it, but once they are available then you can go out and do whatever you want.
With GSM your service is bonded with the GSM chip your given. You can purchase a Go-phone, for example, with a pay-as-you-go service contract, pull out the GSM chip, and use that same service with a different phone.
With CDMA networks it's much more difficult since you'd have to register your phone and configure it to work with a specific vendor. You can't just mix and match whatever you want.
The same would be true for the IPhone (being GSM) if Apple were not complete nazis about it and had the DRM'd up the you-know-what. (of course there are hacks, but whatever)
The thing is, of course, the T-Mobile stuff is subsidized by your service contract. So the 'purchase price' of the phone is really the 'down payment' and you pay installments every month for your phone along with your regular phone bill. So it seems the phone is much cheaper then it really is. (for example: the IPhone's price tag is a simple out and out deception)
Now if you pay full retail price for your phone then you can use whatever service contract you want with any GSM provider. There are special cheaper service contracts and bundles that AT&T and friends won't advertise on their websites or tell you about in their stores. They exist, but you have to poke and prod to find out about them.