Apparently Apple enabled multitasking gestures on the iPad in a 4.3 beta but later removed them... However those that managed to get them have maintained them through future updates (at least up to 4.3.2).
This site http://www.tuaw.com/2011/03/13/want-new-gesture-controls-in-ios-4-3-on-your-ipad-heres-how/ describes a way to re-enable them if you do a collection of steps and buy an app but that seems a bit drastic for basic functionality.
Are there any other free options to re-enable this feature?