1 Rename aac file to .m4r using Get Info (command+I)
2 Double click file to open in itunes
3 Close itunes
4 Goto username/music/itunes/itunes music/ringtones
5 Rename .m4r to .m4a again using Get Info
6 Sync phone
Make sure you name them the way you want, as you cannot remove ringtones (from the iTunes list) once they've been added.
For those having trouble getting this working (and are on a Mac), we recommend iToner.
AppleGazette posted iRing which is a script (for Mac) that will perform the above automatically. Make sure you get the right version (one for iTunes 7.4, another for iTunes 7.4.1)
Update: This post details instructions for how to accomplish this on Windows XP and Vista.