Speak now about this change or forever hold your peace... Satoshi will be including this functionality in the next version of Bitcoin (0.3.12).
If you use the JSON-RPC api, you should check your error-condition-handling code; again, the changes are that the error member will be an Object (with 'code' and 'message' fields) instead of a String, and the HTTP status code may be 404 instead of 500 for method-not-found.