Now, there was a forum post for this before but that one was back in October and wasn't necessarily correct.
But, if you're not one of those people that keeps up with news on like TmoNews or etc...
T-Mobile has announced that they are spending 4 Billion rolling out LTE and REFARMING a lot of their 1900Mhz Spectrum to UMTS/HSPA/HSPA+
The refarm is already happening, and should continue massively throughout the next few years (though is mostly going to be done THIS year),
and not that you guys care as much but LTE will be released next year.
But the main point of this is, if you DIDNT already know, the 1900Mhz band is NEEDED to run 3G/4G on the iPhone.
So get ready for all your "E" symbols to change to "3G".
Also, if you have 3G disabled, I'd recommend periodically throughout this year switching it on and seeing if the 3G signal comes in. and if you have your APN for data set to "wap.voicestream.com" please change it to "epc.tmobile.com" to get full speeds.
If you live in a city you'll probably get the refarm faster then those who don't.
It's also not confirmed which EXACT locations this is happening to, but will most likely be the majority of locations where 3G/4G coverage is ALREADY.
Any other info you need I'd be happpy to try and help and if I can't I'm sure other people on this forum can. 
-Josh.