First time I have used an I pad, someone had left safari running. First thing I did, google "iPad alt tab".
http://noscope.com/journal/2010/01/why-the-ipad-doesnt-have-multi-tasking suggests the home button which seems to be the hw button with the square on it.
Quality of a netuoy us bot sttauobed.
PS: In case it wasn't obvious, I did all that on an iPad. The post is a little unclear because typing was so much work.
Edit: Added PS using a real keyboard, so much nicer.