[GadgetKen]

Sounds intriguing if United Mobile gives out the sims either free or for a small charge to cover the sim card cost and shipping. Worth a try.
Hopefully they will have accurate billing for this; my +44 UM+ calls from the UK to US were mischarged as more expensive Russia to US calls in late 2007 and I had to contact customer service to have them credit the difference (it was interesting trying to convince them that I was across the street from Kensington Palace rather than the Kremlin). Right now I will depend on 2 US MVNO cards for calls placed in the US. For outside US would use Digicel in non-US Caribbean. For outside the US or Caribbean would use MaxRoam or in some instances UM+, Iridium or Skype. Less complicated than it sounds since I only travel outside the US a few times a year, and not many people need to be provided with the mobile number(s) where I can be reached.
Another possibility is to get a dual or tri sim multiband phone (thinking more of the GSM only phones rather than the GSM/CDMA hybrids) which have been dropping quite a bit in price lately. These are mostly made in China but are exported through various sales channels; some have interesting add-on features like FM radios/TV's/MP3 players).