All you have to do is edit a plist file and you'll be up and running. Which plist file to edit is dependant on which mouse you use, but if you don't use an Apple or Logitech mouse you might have to scramble a bit to get it working. For complete details, check out the tip on MacOSXHints. Of course you're still going to need some sort of remote to actually use Front Row, and there are a number of options available for that (a Salling Clicker-enabled cell phone, for example), but being able to activate it is a good first step.
I don't have a Mac Pro handy to test this with, so let us know if you get it working on yours!
UPDATE: We've added new instructions for users of the Bluetooth Mighty Mouse. See this post.