With yesterday's release of GarageBand 4.1.1, you can now "export your original song, your original audio recordings, or use Apple Loops and iLife jingles to create a custom ringtone for your iPhone," according to Knowledge Base article #307108.
The process requires GarageBand 4.1.1 or later, iTunes 7.5 or later and an iPhone with software version 1.1.2 or later.
Apple says that to create the ringtone, you need to "set up a cycle region that covers the area you wish to use in the GarageBand song. Note: This area must be 40 seconds or less in length. Once the cycle area has been set, choose Share > Send Ringtone to iTunes."