At this time, the only way to use custom ringtones on the Apple iPhone is through a somewhat complicated hack.
Apple is rumored to be planning custom ringontes for a coming update to the iPhone.
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Nothing complicated to it, if you use a mac. Open up iTunes and Garageband. Create a new track in Garageband, and then right click on the sound track at the top and delete it. Drag the track you want from iTunes to Garageband and clip it to under 40 seconds, set it to fade in/out and loop. Click "send ringtone to itunes" in Garageband and you're golden. Decent guides: http://chris.pirillo.com/2008/07/22/create-ringtones-for-your-iphone-using-garage-band/ and http://www.brighthub.com/mobile/iphone/articles/6784.aspx .