AT&T and T-Mobile felt that the Galaxy Nexus wasn't worth bringing to either one of the carriers. Thankfully the guys over at I-Wireless have brought an HSPA+ Galaxy Nexus to the states. I Wireless is an affiliated network with T-Mobile and is offering this great device on sale for $199.99
Since I Wireless is affiliated with T-Mobiles network and uses its nation wide service, you can order A Galaxy Nexus from I Wireless and switch the sim card with your T-Mobiles sim card. This method may also work for AT&T sim cards as well.