The iPhone's lack of availability on T-Mobile is actually a technical limitation, because T-Mobile uses a very unusual frequency band for 3G that most phones, including the iPhone, don't support. They are changing over to something more common, and will likely get the iPhone soon because of it.
Not to argue with anything you're saying, just pointing out that that particular decision was more technical than anything.
Definitely a past tense thing, and there isn't anything stopping the sales of iPhones now. Heck they claim there are already ~2 million iPhones on the network already!
The "unusual" frequency is also known as AWS bands, and are also used by other carriers and in other countries (only 4 at the moment so not pervasive), although there are now LTE deployments happening in the AWS bands.
When tmobile does start selling the iPhone (rumoured to be a few months out) it will be interesting to see if they piggyback off the parent company or deal with Apple directly. In order to deal with Apple you have comply with minimum order quantities (remember the Sprint $20bn numbers) http://www.asymco.com/2013/01/17/the-iphone-moq/
Well, people were using iPhones on T-Mobile from day 1. It always worked, but 3G didn't. So, while "working", data speeds were excruciatingly slow.
(I actually went the opposite direction for a little while. I had a Nexus One made for T-Mobile which I used on my AT&T account. It had the same problem in reverse, since it only had T-Mobile's 3G frequencies. The phone itself was decent, but the data speeds were a killer.)
Anyway, good to hear that it's happening, and thanks for the links. More competition is good, and I'm sure that a T-Mobile iPhone will help.
I had an iPhone 4s on T-Mobile in NYC for about 2 months late last year (I think October–mid December or so), and the whole time, saw a 3G signal about one time (a particular block in the West Village, IIRC.) The rest of the time I was stuck on edge.
Not to argue with anything you're saying, just pointing out that that particular decision was more technical than anything.