How to create custom ringtones in GarageBand 4.1.1  —  With GarageBand 4.1.1 you can export your original song, your original audio recordings, or use Apple Loops and iLife jingles to create a custom ringtone for your iPhone.  —  Here’s what you need to do it:  — GarageBand 4.1.1 or later

Source:   Apple

Link:   http://docs.info.apple.com/article.html?artnum=307108