Answer: when T-Mobile uses the word "UMA" as its network ID for all roaming customers, instead of "T-Mobile US".
No idea why this happens - initially on an unlocked Nokia E71 with a T-Mobile SIM (I thought it maybe had a "hidden" UMA stack and auto-logged on to the SFO Airport network, until I switched off the WiFi). Then I got the same tag on my non-WiFi SonyEricsson, and realised the answer must be simpler.
I'm pretty sure it's always just said "T-Mobile US" when I've been here in the past.
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