Yesterday Apple released a minor update to GarageBand, the minor update appears to have included an undocumented major feature; the ability to create ringtones and send them to an iPhone.
An Apple support document details the procedure.
According to Apple all that is needed is an iPhone with Firmware 1.1.2 or above, iTunes 7.5 or later and GarageBand 4.1.1.
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1. To create a custom ringtone in GarageBand follow these two small steps:
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.
2. Once the cycle area has been set, choose Share > Send Ringtone to iTunes.