GTK+ 3.0 will be available for 11.04, but not included by default and none of the default applications will be built with gtk3. This will give application developers time to port to gtk3, theme engines and themes to be updated, etc.
I like the idea OP, and I'm interested in the answer, but for most uses you can utilize 'natural scrolling' plugins and daemons. You should search for 'natural scrolling' stuff on the web, or if you want you can try Android-X86. It's very fun.
Best Answer
GTK3 is installed by default, please don't download and compile from the GTK website.
If you want to use GTK3 yourself, compile against that instead GTK2, or use PyGObject instead of PyGTK if you're using Python.
If you want all programs to use GTK3 instead of GTK2, it won't work, it's a job for the application developers to port their application.