It's only the iPhone really.
There's a 1700MHz (AWS) version of the Samsung Galaxy S3. There's an AWS version of the HTC One X and One S. Nokia's phones are mostly multi-band including AWS. Apple is the only big hold-out, but this costs T-Mobile dearly, yes.
T-Mobile knows this, and is therefore repurposing the AWS spectrum for LTE, and switching its 3G (HSPA+) network to 1900MHz as 2G GSM (that presently occupies that band) declines. This is compatible with what everybody else is doing. AT&T is rolling out LTE on AWS (as well as 700MHz) and so likely will Verizon at some point. The iPhone 5 supports LTE on AWS already, so once T-Mobile does this they will have a network using the same bands in the same way as AT&T, so the same phones that work on AT&T will work on T-Mobile as well.
This merger gives T-Mobile more AWS spectrum (some of it with LTE deployed in it already), and more 1900MHz spectrum. When the CDMA network being used by MetroPCS is phased out, they will be in a position to have extensive 3G and 4G networks that are compatible with everyone else.
In the short term, T-Mobile will post-merger have a situation where former T-Mobile customers and former MetroPCS customers will be roaming from different 3G networks to the same LTE network, and at some point where the CDMA 3G network of MetroPCS will have to be switched off without too many customers losing coverage. That will be a good trick if they manage it.