An updated step-by-step to use your own Ringtones in iTunes 7.4.1 (thanks JPyre)
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
Done, simple, works everytime, 100% consistent. Make sure you name them the way you want, as you cannot remove ringtones (from the iTunes list) once they've been added.
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