If you have (or want) an iPhone, you used to be locked into AT&T’s 2 year contract. However, hackers have released a free unlock which allows you to use T-Mobile as a carrier. Actually, you can use any carrier that is using CDMA, but in America, that is only T-Mobile and AT&T.
Verizon is a no go, because it uses a different technology. But the rest of the world (that use CDMA) is now a very happy place. So if you like T-Mobile and want to buy a nice gadget, go nuts. Oh, and make sure it’s free. Other hackers were trying to charge for it, but the free project released before they were able to effectively mass-market it.
Details (from Gizmodo) are here.