By TheGeek on July 31, 2009 via http://www.demogeek.com/2009/07/31/how-to-add-custom-ringtones-to-your-iphone/
The courterpart Windows guide is: http://www.softdiggs.com/howto/create-custom-iphone-3gs-ringtones-using-itunes.php
Note: If you want to avoid the trouble of making iPhone ringtones following the steps below, you can get this Mac iPhone Ringtone Maker to help you get ringtones for your iPhone on Mac OS.
Easily add custom ringtones to your iPhone using iTunes
The default ringtones that comes with the iPhone are pretty basic and blank. You can, instead, use your own piece of music as a ringtone on your iPhone. All you need to get this done is …