The easiest and safest way to do this is with the iTunes Music Store. It’s also not free. The music industry believes that you should pay extra for a song you already own if you want to use it as a ringtone. Needless to say this is absurd, but it’s the only official way to do it.

However, there are unofficial ways, but there’s always the risk that Apple will modify the iPhone in the future in ways that could not only make this technique fail but could even damage your phone. Apple may also refuse to support your phone if you have modified it in any way.

IToner is a relatively non-invasive way to do this. It’s not free - it’s $15 - but it works well and is unlikely to permanently harm your phone.

As we record this (with iPhone firmware 1.1.1) you can also install free apps on the iPhone, including Erica Sadun’s Song Mover, by “jailbreaking” it at jailbreakme.com - you do this at your own risk.