Metro has a tiny, but growing 4G LTE network, so a future WP7 phone is possible, but I would expect them to be one of the last to get on board. Generally they focus on low cost first, so they keep their line up simple, that means they probably aren't going to rush to fit in a WP7 phone when they are not in huge demand. They don't even carry an iPhone, which is very popular even though the could fit a 3GS in their lineup at their unsubsidized prices.
I'd expect them to carry the iPhone first before a WP7 phone, but who knows. They might pick up the Lumia 6xx or 7xx at some point.
I left Metro PCS to buy my Lumia. I've had it with Android. I had the "top of the line" LG Esteen Metro offers and it was a horrible buggy unreliable piece of junk. The 4G LTE was great for a while.. until I could barely connect to it most of the time. Also with Metro, there's no 3G, so you are either in a 4G LTE area, or you have horribly slow, virtually unusable Internet.
That $60 a month bill was nice, but ultimately the complete lack of support from clueless store employees (far worse than any normal carrier) and the crap selection of phones just drove me away.
If you want a no contract, less expensive carrier with a Windows Phone, I suggest looking at T-Mobile. You should be able to get a Lumia 710 or a Samsung Focus from them with no contact for right around $70 a month.