You have no choice but AT&T for iPad 3G service. There are other threads on tethering your iPad to a smart phone such as an iPhone, or a Droid, etc to get some connectivity to the web. But as is, the iPad 3G service provider in the USA is AT&T.
In Canada we have lots of choices (Bell, Rogers, Telus, SaskTel), but alas, only AT&T in the US.
T-Mobile will work on the iPad but there are no iPad plans; you'd need to buy a standard data plan. Also, the iPad doesn't support T-Mobile USA's 3G frequency (1700 MHz) so you'd be using 2.5G EDGE.