It just did it! I thought I might have touched something, but I can't work out what I could possibly have one.
I've checked that it's not inversion, have tried a reboot (holding home and power buttons down) and, after googling for advice, slapping it. But no joy.
I don't really want to try taking it apart! And I'd like to avoid a trip to the Apple Store tomorrow if at all possible. Any other ideas??