It should be available with all operators ideally but initially Apple will play with deal with AT&T only as they do it every time whenever a new device is launched. May be later on it will be available with other operators.
Not really... T-Mobile has less coverage than AT&T and they throttle you down to EDGE speeds once you hit 2GB of data.I thought they'd make a world phone, but with the iPad 2 having both AT&T and Verizon models, I'm not so sure anymore.
If it does come out on T-Mobile, there's no point in selling themselves to AT&T. Customers would probably flock to them who want to use GSM/HSPA.
Not really... T-Mobile has less coverage than AT&T and they throttle you down to EDGE speeds once you hit 2GB of data.