Thanks to RS, I have a better-late-than-never story about GSM roaming in the US. A press release from AT&T Wireless indicates that a deal was struck in April that would allow joint roaming between the AT&T and T-Mobile GSM networks starting in June of this year.
The fine print is that they don't appear to be activating roaming in places where their coverage areas overlap. Thus, don't expect to get further in to Great Falls or better coverage in New York City, but you can expect to have better coverage in rural areas that were not previously covered.
Further, the two have agreed to roll out their new cells in a coordinated fashion, allowing them to leverage each other instead of competing for providing rural access.