TUAW’s Erica has released the new iPhone application, Song Sender, which allows users to send their songs via email and set them as ringtones with a single click. Once the application is started, it will read your library and create a scrolling list of all the music on your iPhone. Tap the song name, and from there you can play it, email it, or add it to your ringtones.
The Song Sender is a free download, and is one of those apps which should have come pre-installed with the iPhone. Anyway, the unofficial solution is here, and many who have tried it found it quite usable!