Bought my Mini at the Apple Store back in April. I was planning to do wifi only in the US and have the capability to buy data when I travel. I got the ATT model. I did not have to activate it on the ATT network when I bought it. The nice thing about buying it at the Apple Store was that I got to have one of the people there give me some individual time to set up there and set up the syncs with my MacBook Pro and Itouch. I used just wifi the first month. When I got over to Italy in May, I went to one of the TIM stores and bought a SIM card and some data. Popped it into the Mini, activated on TIM, and it worked great. Got back home, just kept cellular off and use wifi.
No muss, no fuss. I'm a T-Mo person, so I may eventually sign my Mini up with them for data. But, I'm not in a rush for that...