We're changing the response code sent back by the Search API when the rate limit has been exceeded. At present, it is impossible to distinguish rate limit responses from other error conditions in responses from the Search API -- this is what we're trying to fix.
Starting Monday, January 18th, 2010 the Search API will respond with error code 420 in the event that the number of requests you have made exceeds the quota afforded by your assigned rate limit. Please update your response your response handler to accommodate this new behavior. Apologies for the false start last time this change was announced. If you have any questions, please feel free to post them on twitter-development-talk. Thanks!
