Easiest is to just hop into a T-Mobile shop at your destination and get one. They're easy to get.
The only caveat is that some areas do not sell native T-Mobile service, e.g. in most of the Dakotas you can't find the kits. If the area you are visiting has native T-Mobile phone numbers you should be good.
GSM: SaskTel Canada (postpaid voice + data), Rogers Canada (postpaid data), T-Mobile US, AT&T US, Red Pocket Mobile US, Orange UK, 3 UK, Swisscom Mobile Switzerland, tele.ring Austria, airBalticcard (Estonia)
CDMA: PagePlus US
GSM phones: iPhone 4, iPhone 3G, Nokia Nuron (AWS 3G), Otech F1 (quad-SIM 2G GSM), Nokia E63, Sony-Ericsson K610i, Sony-Ericsson Z310i. All unlocked.