The unlocked version is not tied in any way, shape, or form to T-Mobile.
If your in the USA you can use it with AT&T if you want, or the small handful of other GSM using cell phone service providers.
Cell phone providers outside the USA are generally all GSM and use frequencies compatible with the G1. (check your local area..) So you can use your current provider or cell phone plan or any of the huge number of pre-paid phone/data cards available to folks outside the USA.
Anyways... Freedom isn't free. It never has been, nor will it ever be. So if you give a crap about having a open system to run your software on then the strongest signal you can give is to put your money were your mouth is. If it's not possible that is one thing, but it usually is.
T-mobile's plans, are in effect, trading you discounts for Freedom. How much are you really worth? Like I said it's one thing to not have choices, but it's another to have choices and pick simply based on cost.
I am not going to by a Android phone that is locked into a service or requires special signed firmware. It's just not worth it.
Buying a locked-down Android phone is worse then buying a iPhone, as far as a end user is concerned. You don't get any freedoms and the interface and the available software on the market isn't as good. If your going to buy a pair of handcuffs you might as well get the shiny pair.