5
It sounds like a great idea to me, but why don't we take it one step further. Allow the user to select the type of referral, ie. Search Engine, Registered Member, etc. IF the user selects "Search Engine", display a drop-down menu where they could select the search engine, or for "Registered User" display a drop-down menu with a list of members. The only drawback would be if you have a large member base. :(
Chris