Just as I was starting to get used to the four- and five-finger gestures in iOS 4.3 beta 1, Apple releases beta 2 along with this little note regarding the gestures:

This feature will not be enabled in iOS 4.3 for customers, but we are providing this preview to gather input on how these gestures work with your apps.

My best guess is that Apple isn’t ready to release the gestures just yet, but are planning to do so in a later version of iOS. The developer preview is more or less a subtle nudge letting developers know that they should be ready for them.