yes, the Blackberry pearl can be used on the AT&T network you can even watch a video clip, listen to the songs you like. you can use all the features of blackberry in AT&T network.
Yes, T-mobile and AT&T Both use GSM Networks (Sim Card) It's possible to switch between the two companies but you will need to unlock your blackberry pearl. If you have been a T-mobile costumer for more than 6months you can contact them ask for the unlock code that corresponds to your phone.
I am assuming that you want to switch from T-Mobile instead of T movile. If that is the case, the Blackberry you had with T-Mobile will work on the ATT network as long as the phone has been unlocked.