I've searched for this here and don't see it. I've set a custom, contact-specific ring tone before but don't recall how. It doesn't seem to be under the contact, which is where it should be.
Phone is a 2.x.
OK I found it:
Go to the Contact.
Do not press Edit.
Hit Menu.
Hit Options.
Select Ringtone.
Set Ringtone.
Seems wacky that you don't edit the contact to change the ringtone.