Unfortunately, your iPad 3G is not capable of HSPA+ network.
T-mobile's 3G works on 1700 HZ frequency and not AT&T's 3G's 850HZ frequency.
Of course, this transition will not happen over night. It will take longer than a year that both AT&T and t-mobile claim...
It is very likely that AT&T will deploy HSPA+ network for this year and get ready for LTE until 2013 to battle with Verizon.