Multitouch gestures don't work on the original iPad, even though they worked for developers on iOS 4.
I guess this is either to sell more iPad 2s, or because the performance when switching apps is slower (it is), which makes the app switching gesture worse.
Either way it's quite a dubious decision IMO. iOS 5 is all about being able to do more with the iPad. I've been using it a lot while writing my dissertation, and the main issue is how slow it is to switch between safari/iBooks and the writing app. The horizontal tabs in safari are a great improvement in this sense.