Well, I never did it by myself but if I would be tasked I'd tried the following:
1) Download this: http://techdhaan.wordpress.com/2008/05/09/sharepoint-search-web-service-query-tool/
2) In the query interface chose somthing ( I dont remember) about exclude duplicates.
3) connect responce xml from Search.asmx to your form.
4) Populate dropdowns with Search results.
Hope it will help. Let me know if you were successful.
P.S. To optimize it even more you could resend your updated query each time as new dropdown value is populated.