I worked my way through Worlds 1, 2 and half of 3, then had to put the iPad down to answer the door. When I started up the game again, levels 9 and 10 in world 1 were locked, even though I had already played them, and world 2's levels were all showing locked and unplayed as well! World 3 was completely locked, like I never unlocked it. Gamecenter clearly shows my achievements, so what happened to my saved in-progress game?
If I'm expected to play this through in one shot, then this is getting deleted. As it is, it's frustrating enough to lose most of my progress that it's going to be a while before I play this again!