Does Ubuntu support the full functionality of the Apple Magic Mouse? What bluetooth adapters work best? What are the caveats?
Please see this to get help with MultiTouch in Ubuntu: https://wiki.ubuntu.com/Multitouch#Community%20Help
There is a section dedicated to the Apple Magic Mouse, I also have that device and am interested in documenting any multitouch information there.
The above page also includes information on testing and using demos for multitouch.
If you want to see the current gestures available in Ubuntu 10.10 you will need to install the Netbook edition (Unity). On a standard desktop you don't need to reinstall, just add the ubuntu-netbook package, logout, and login again making sure you choose Ubuntu Netbook Edition at the bottom of the login screen (after choosing your user).
Ubuntu supports using it as a standard mouse (move the cursor and left/right clicks). As far as I know, all bluetooth adapters work out of the box.
The driver updates in the next version (Maverick, 10.10) are expected to support scrolling and might support multitouch.
According to this forum post someone has backported the scrolling support to Lucid, but it doesn't appear to be available in a PPA yet.