In an effort to simplify our APIs, we are standardizing the response codes returned by our various systems. Historically, the Search API has returned 503 for rate limiting whereas the REST API has returned 400. So, we are changing the response codes sent back from the Search API.
Starting Wednesday, December 16th, 2009 the search API will respond with error code 400 in the event that the number of requests you have made exceeds the quota afforded by your rate limit. Please update your response handler accordingly. If you have any questions, please feel free to post them on twitter- development-talk.