When the multitouch trackpads first came out on the Macbook Pro, someone wrote a great app/preference pane called Multiclutch that allowed customization of the gestures.
Interestingly, when I use a Magic Mouse with my Macbook Pro that has Multiclutch installed, some but not all of the customizations port over. So, for example, I remapped the three-finger left swipe for the trackpad, and that mapping carried through to the two-finger left swipe on the Magic Mouse. I suppose this means that there is technically a way to customize the multitouch on the Magic Mouse, if someone is interested/able to write a new app for it.
Interestingly, when I use a Magic Mouse with my Macbook Pro that has Multiclutch installed, some but not all of the customizations port over. So, for example, I remapped the three-finger left swipe for the trackpad, and that mapping carried through to the two-finger left swipe on the Magic Mouse. I suppose this means that there is technically a way to customize the multitouch on the Magic Mouse, if someone is interested/able to write a new app for it.