I have the same problem on my iPhone 3G - it happens when I am out of the game using another application, and then return to find my game gone and the startup screen there - and it's always when I am well on in the game, as well. Very frustrating. I do not mess around with the time as I always play at double speed from the start, and it has never happened when I am actually doing anything in the game, only when I am out of it.

I look forward to a patch to correct this ! It seems to be happening more recently - twice in a few days.