iTune 9 seems not to allow for users to add aac or m4a format to iPhone ringtones. When I added m4r file which is changed its extension from m4a, iTune didn't allow to add it. So, probably old technique to add ringtones would not work anymore.
However, iRinger did really good job. Furthermore, it's free. Instead, it offers some ad in its program.
For more information: http://www.iringer.net/