Even though T-Mobile took a lot of precautionary measures to make sure that the phone wouldn't be able to be rooted, the hardware is now fully unlocked. This is more than just simply rooting and putting ROMS on, the hardware is now completely open for development.
Keep in mind that it's a very complicated process done by some pro developers who know exactly what they are doing, but I'm sure they'll post a step by step guide on how us normal android users can do it. The guys over at XDA definitely deserve a round of applause for this feat, since it definitely wasn't easy getting past that many restrictions and security issues on the phone. Check out the link below on how you can do this(not recommended unless you're truly experienced at rooting your phone) and also check out our T-Mobile G2 Review.
Via [XDA Wiki]