Yes, iPhone and iTunes are buddies, very good; but when it comes to adding ringtones to your iPhone toy, iTunes will not let you, complaining about purchased MP3 content through the iTunes store. In other words you will not be able to add MP3 's to your iPhone if you did not purchase them through iTunes.
Well, not anymore, thanks to iToner, the software from Ambrosia Software gets the job done, without hassle.
Your iPhone allows you to assign different ringtones for your different contacts. With iToner, now you can have a Barry White ringtone for when your girlfriend calls, Frank Zappa for when your college buddies ring you up, or the theme from Jaws for when your boss is calling.
Download the software here and purchase it at only 15 $, the demo version will work for 1 month, so you can test it before buying.