Here is a blog entry I dound on ipodobserver.com:

Ambrosia Software announced the immediate availability of iToner 1.0 on Friday. iToner is a Mac OS X-based application that adds custom ring tones to Apple’s iPhone without requiring any special hacks.
The application sports a drag-and-drop interface for copying MP3 and AAC from a Mac to an iPhone where they appear along with the pre-installed ring tones. Since it does not require any changes to the iPhone or its code, ring tones will remain accessible even after iPhone software updates.

iToner is priced at US$15, and is available for download at the Ambrosia Software Web site.