I have a MaxRoam SIM.
Not sure I can recommend it though. It works great in the U.S., great in Mexico, and doesn't work worth shit in Costa Rica. Outbound call quality is terrible, and in-bound calling almost never works.
CelTek also offers the service, but given my MaxRoam SIM says Celtek on it, I'm guessing they may use the same infrastructure.
I bought it because I've been waiting almost 1 year for a GSM phone line for the phone I bought my wife.
I gave up her TDMA line. You can't buy a TDMA phone new anywhere anymore, and the old phone was stolen. My mistake considering waiting for the GSM line.
You can probably find someplace to buy a "recycled" TDMA phone with receipt acceptable to ICE, assuming they are still activating TDMA lines (they ought to have many available). TDMA service is supposed to be dropped very soon, but I imagine people will complain and they will extend it.