From the begining Android apps are closed/crash after sent to background by anything else.
BUT yesterday a funny thing happened to me.
My AndroTouch

device (ICS) was running Digital Photo Frame. I touched some on-screen key (I think it was the |^|) and my app was sent to the ICS BACKGROUND. I mean, my app was running as a background image, with all the Android icons over it. It was so funny, the photos moving/displaying in the background, with his movement and all of this. Also I can not stop it, I had to restart the tablet.
I tried to replicate it with no luck.