No. You have to already have your .m4a ringtone built. Rename it to an .m4r file, and add it to your iTunes library. When you sync your phone the next time, it will show under "Ringtones" on the iPhone tab. It doesn't actually show up in the library after you add it.
Yeah, Erica's been doing a fabulous job, despite hecklers in her comment lists. This tip actually comes full circle through, she links to the MacRumors write up, which began from the post I made in the forum here. Kudos all around. If you look at the original thread, its got an enormous amount of feedback in it for both Mac and Windows.