Someone recently posted on the Spotwalla mailing list that they were having problems with their spot when the visited other locations. I don't recall the details, but it involved travelling large distance in a quick timeframe, and I think that they were a pilot, so they had it on while flying. They contacted Spot and were instructed to do some sort of reset and all was well.
I know that's very vague, but Spot may be able to assist you, if your situation is similar.
I've used mine overseas for brief stints with no issue, but I don't know the intricacies of the unit, so could have problems one day.