First the iPad was jailbroken, now it appears to have succumbed to a hack that allows it to be used with a T-Mobile SIM card. Although the iPad 3G isn't carrier locked to AT&T, network differences between the carrier's 3G bands mean that an iPad 3G on T-Mobile will only be capable EDGE speeds. Until today, that is.
PCWorld posted all the gory details on how to use a T-Mobile SIM in an iPad, but beware, the hack involves cutting a traditional SIM card with scissors (being careful not to cut the gold contact) to turn it into a micro SIM (below). A couple tutorials are available on micro SIM conversion.
In related news, a video (embedded) from the iPhone Dev Team shows a jailbroken iPad 3G sending SMS messages from T-Mobile. Although it isn't pretty and requires terminal commands it can't be long before someone writes a nice UI and distributes a handy app via Cydia.