Just bought a T-mobile sim card on E-bay and popped it into an Ericsson T39 unlocked tri-band I had also just bought on E-bay. Popped it in and it works.

The cost was something like 8 bucks (plus shipping, how they can charge another 6 for putting an item into an evelope is beyond me), so about 15 bucks including California tax for 150 minutes.