Open the AVI file with QuickTime Player, go to the File menu and choose Save As..., and then pick "Reference movie" as destination format. QuickTime Player will generate a small MOV file that acts as an alias to the original AVI movie -- and this reference file can be added to iTunes. iTunes will let you add metadata to the reference MOV file, and will play the original AVI if instructed to do so. Note that if you remove the original AVI file, iTunes will obviously not be able to play it, even if the title still appears in its database. Also, reference movies are still somewhat big for what they are worth (around 1 to 3 MB each).
Let me add a side-hint for those that have set iTunes Preferences (Advanced category) to copy files added to the Library instead of using the original file. Since original AVI movies will still stay in their place, while original MOV movies will indeed get copied, and you probably do not want to have huge files duplicated or your movie library split in two, you might want to leave all your movies (be them AVI, MOV or whatever) organized by hand in a place of your choice (~/Movies?), and press Option while dragging MOV files to the Library, which temporarily overrides that option in the Advanced Preferences.
By working this way, you will be adding the movies to the iTunes database without making a duplicate of the MOV file, nor moving it to the iTunes Music folder. Be aware, though, that adding metadata to original's hardwires that metadata into those original files (i.e. modifies them). Not that it should be a bad thing, but just so you know.

