How to record a new voice mail greeting on your iPhone.
1. Tap Settings.
2. Tap Voicemail.
3. Tap Greeting.
4. Tap Custom.
5. Tap Record and record your voice mail greeting. Tap Stop when you are done.
6. Tap Play to preview your greeting.
7. When done, tap Save.

This is really helpful but does anyone know how to upload an MP3 as a voicemail greeting?