You can already do this in iOS (I think iOS 5+). Just go into a contact, tap on "Edit" in the upper-right corner, scroll down to the "text tone" option, then tap on that and select the desired tone for that contact.
You can already do this in iOS (I think iOS 5+). Just go into a contact, tap on "Edit" in the upper-right corner, scroll down to the "text tone" option, then tap on that and select the desired tone for that contact.