Clarification of http-409

Hi list.

There is a question on stack overflow [1] about the proper status code
for a response where the server side validation fails.
The best suggestions seems to be 400 or 409, but is 409 really the
proper response in this situation?

[1] http://stackoverflow.com/questions/2190945/what-http-response-code-for-rest-service-on-put-method-when-domain-rules-invalid

-- 
troels