When combined with an unlocked GSM phone that's also WiFi capable, Cubic Telecom's MaxRoam service shows some promise to dramatically lower the cost of roaming not just for international travelers, but also for those who want to reach him or her.
In addition to getting a SIM card that must be paired with an unlocked phone, MaxRoam users also get access to a Web site that's essentially a dashboard for controlling their phone.
Through the Web site, you can pre-pay minutes for your SIM (TopUp), buy additional phone numbers that will ring through to your phone (over WiFi and/or the cell network), manage existing numbers, and forward any of the numbers under your control to another number (eg: a landline or another cell phone -- perhaps a CDMA-based one when you're in the States).
For David Berlind's write-up on MaxRoam, see his post
in ZDNet's TestBed blog