T-Mobile will allow you to utilize an unlocked iPhone on their network. From what I've heard, they still won't be selling the iPhone but you can buy an unlocked iPhone from an Apple store or Apple.com, and then go to the T-Mobile store and pick up a micro sim card and activation kit along with purchasing your pre-paid minutes for whatever plan you opt for and you're in.
When you get the phone working on T-Mobile, you will have to tweek your settings in order to get and receive picture & text messages, but that part is simple and if you either Google or visit the
T-Mobile site they have instructions posted on how you can adjust this.
I speak from experience because I recently had a iPhone an unlocked iPhone 4S on T-Mobile that I purchased from Apple.com, I ended up changing my mind and returning the phone back to Apple because the T-Mobile data service was slow in certain spots for me and in certain instances I had no building penetration to use the phone inside to talk (i.e., the mall, Walmart, etc.)
Still don't let my experience stop you, they're fixing their towers as we speak and should be carrying far better service in the coming weeks especially with more iPhone users wanting to come to
T-Mobile for the no-contract deals.
iPhone 4S is $549 iPhone 5 is $649; like I said your best and safest route is to get the unlocked phone through Apple.com or Apple store. Don't choose ebay or amazon to purchase your unlocked phone, since I've heard that unlocked phones purchased from those sites, will turn to a brick and be unusable if you attempt to download the latest software update -- which in turn comes out to be a waste of money.