Getting a phone number through itunes?

So basically im on a prepaid att phone account right now because i wanted to wait for the iphone to come out. now my problem is i tried preordering but it wont let me because its a prepaid account. and i cant switch to contract account while preordering the iphone at the same time.

so if i get a new number through itunes, do i get to choose from a list like i can at an at&t store??

 

oh and if i start a plan with my own phone does that mean im not on a 2 year term?