Not only apps ....
I remember downloading the pop video for Captain Jack by Captain Jack not long after the iTunes store opened. It was a wee bit dodgy (had some boobs in it) and one swear word but nothing that bad.
A couple of months later when I synced with itunes again they had removed the video from the store (obviously finding out it was a bit ropey) and it automatically got deleted from my devices as well, even though I had paid for it.
Luckily I had converted it to another format and kept a copy but it just shows that apple even have the power to remove apps or media without telling you once you have bought it. Bastards!