I went to the Apple store to mess with the trackpad but the 3 finger and 4 finger can only do one thing each, expose and app switch. Will there be a hack to it or a 3rd party app that will let you customize the multi-touch? For instance, use it for spaces or dashboard or something else.
None that I'm aware of. Multi-touch doesn't even work in all apps or many for that matter with the exception of increasing or decreasing text size.
Take a look at multi-clutch it lets you assign multi-finger gestures to keystrokes. I don't think the current version supports the new three and four finger gestures. I have a few actions set up for coda and it works pretty well. http://wcrawford.org/2008/02/28/everytime-i-think-about-you-i-touch-my-cell/
I use multi-clutch to set three finger swipes as back and forward. Works in Firefox and in finder. There is a limited list that you can choose from for each function.