You can double-click the home button, or use a four-finger swipe gesture, to pull up the list of “recent” applications along the bottom of the screen, and then tap one. Also, you can four-finger swipe right and left between applications.
The four-finger swipe only works on the iPad. That said, I love those gestures. I feel nearly as fast and as comfortable multitasking on my iPad as I do when doing similar light tasks (web browsing, music listening, IMing, etc.) on either of my desktop operating systems (Windows and Ubuntu).
