Easiest way is to buy the device at Best Buy and then go to a physical T-Mobile store and setup an account. Go prepaid or request the deprecated "Even More Plus" plan - they still can set one up if you ask. Saves you about $10/month.
You shouldn't have any problem with porting (unless you're under contract) or using a prepaid plan. "Even More Plus" plans are still available though no longer advertised and may save you $10/mo.
Also consider the T-Mobile reseller Simple Mobile. $60/mo gets you "unlimited" service.
Yeah, I'm also interested in getting a smartphone on prepaid (or postpaid, whatever, I don't care as long as I don't have to sign a contract). I hear it's possible with this phone, but I'm having trouble finding out all the info.
Though actually, having spent the last week carrying around an iPod Touch in a large building with wifi, I think perhaps I'm better off being unable to access my email.
If you want pre-paid, just call and they'll mail a sim out to you. I did that and ported my ATT number over with no problems. They can't do pre-paid in store, I don't think.