OK just tried it... It works on Front Row. In iTunes, got to Preferences and then Sharing, and choose to share the library. Use a simple password if you want one because inputting it in FrontRow is a pain.
Then on the mini, open iTunes and go to Preferences, Sharing and tell it to look for shared libraries. It will now appear in the sources list.
In Front Row (hold the apple key and press escape) go down to Sources, and choose the computer you are sharing the library of. Enter the password if you have one and it will load the library. This might take a moment (took about 15seconds for my 2TB+ library) and then you can browse stuff to play. Note that the menus are not the same as the AppleTV and in some cases are not as good to use - such as not having the genres menu in the movies section and such.
It's pretty simple to set up really- hope it works out well for you 🙂