MetroPCS will merge with T-Mobile in a few months and will likely carry the iPhone. I'm pretty sure an unlocked iPhone will work too.
After you hit the data limit, MetroPCS will downgrade you to 3G.
http://gigaom.com/2013/01/10/metropcs-rejiggers-its-plans-again-restoring-60-unlimited-tier/

After you hit the data limit, MetroPCS will downgrade you to 3G.
http://gigaom.com/2013/01/10/metropcs-rejiggers-its-plans-again-restoring-60-unlimited-tier/
Metro is also offering two lower tiers, both with unlimited text and talk: a $50 2.5 GB plan and a $40 500 MB plan. After those caps are exceeded, Metro will throttle your speeds, but MetroPCS has a very gentle definition of throttling. According to the company it will slow you down to CDMA 3G speeds, which may be sub-megabit, but are much faster than the slow crawl most other carriers subject you to.