Apple has placed the ability to make (free) custom ringtones, courtesy of GarageBand 4.1.1.
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An Apple support document details the procedure:
To create a custom ringtone:
1. 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.
- GarageBand 4.1.1 or later
- iTunes 7.5 or later
- iPhone with software version 1.1.2 or later