I just got my new GSM phone (previously I had a TDMA phone). I noticed
that in flight it said "Emergency Use Only" on the display. I was about
4,000 feet over SMF airport.


This is not a feature that's specific to flying. What's happening is the
phone is finding a signal on the 800 MHz band, but it doesn't have roaming
permissions - so it can only be used to call 911 (or the equivalent). Since
800MHz generally has a much longer range than 1900MHz, that's why you saw
the message when you were in the air.

With my sprint phone that has dual band capability, I generally get a usable
analog 800MHz signal in the air, but no digital or PCS signal.

- Ray