I'm not sure that you have to take the unlocked iPhone into T-Mobile to activate it. I think a lot of people sign up for Sidekick plans or prepaid. To use an iPhone on T-Mo's network you have to go to settings and turn off 3G then it will work. T-Mo is very aware that iPhone are on there network. A while back there was a change of some sort that broke voicemail for iPhone users and T-Mo fixed it for them. So they know and more or less offer support for these illegally (now legally) unlocked devices.

It's funny how we needed official clearance to jailbreak our iPhones. Imagine buying a new computer and being told you can't install software that Windows or Apple or Linux etc did not approve of. Imagine not being able to install a new operating system if you wanted to. It's mind blowing and it seems the only right thing to do is force Apple to allow unsigned software to be able to be installed without the need to jailbreak. If I'm willing to accept the consequences they should let me do it.

Keep in mind that just because it's now legal to jailbreak don't think Apple won't continue to release updates that break our jailbreaks.