Rogue Amoeba has released MakeiPhoneRingtone, a free tool that automates this trick for getting AAC tracks into the Ringtones section of iTunes and syncing them to the iPhone freely. You simply launch the application then drag an AAC icon over its main window, and the file will automatically be sent to iTunes as a ringtone, at which point it can be immediately synced to the iPhone. [Note that this tool may be susceptible to changes in future iTunes revisions that could block its functionality]
The company also happens to offer Fission, an audio editing tool priced at $32 that is great for cutting down tracks to create ringtone-worthy snippets. Rogue has posted a tutorial for using Fission to create ringtones.