i have tmobile as well and love it...but here's the bad part though :/
tmobile doesn't have a contract with apple, so therefore tmobile does not sell the iphones.
if you go to verizon or sprint, you will see that the iphones are $199. this is the price after you sign a 2-year contract with those companies, who are conveniently contracted with apple...who is getting money off of all this. however, if you don't use those two companies, then you have to get an iphone that is "unlocked." (note: NOT jailbroken.) and because there's not contract involved, apple will make you pay the full price for the phone, which is $649 through their website and in stores.
there are other alternatives, like amazon or ebay...but just make sure that you get one that is UNLOCKED. on those sites, they'll offer both the old and new ones. after you get a phone, you have to take it to tmobile and have them cut down your sim card, because the sim cards required in the iphone are like half the size of the tmobile cards. and also remember that you won't get the "4G" or super fast internet on the iphone...it will still be the same tmobile internet and plan.