According to a number of reports on the web, a mighty mouse will work. If your Mac is a MacBook Pro (or a MacBook?), tapping the trackpad with two fingers will also work (it emulates a right-button click) provided you enable the behavior via System Preferences > Keyboard & Mouse > Trackpad > Tap trackpad using two fingers for secondary click.
Most third-party USB mice will also work (both in Mac OS and in the Ubuntu guest). For example, I'm using a Logitech MX310 mouse, and it works perfectly in all three operating systems (the Mac OS X host and my Ubuntu and Windows XP guests).
Last edited: Oct 26, 2006