DW had a green card for 6 months before we made the move to England 3 years ago. We had originally applied for her greencard in Arlington, VA, boy was it a pain to go there!
Our move to England was permanent, so after her 1st or 2nd trip back to the States they took it away from her. We said we didn't live in the US, didn't have a residence there and they took it off her on the spot and entered lots of stuff in the computer system and said if we did come back to live in the US then there would be records - I guess the immigration officer was suggesting it might be used to regain her greencard more easily? dunno.
Where we were it was potentially more than a year to get citizenship, something she wasn't sure about getting anyway so that idea was abandoned and now I'm getting my British citizenship on Wednesday!
Matt