Just to round this out -- one other case where you could get a 503 response from your site was if it had gone to sleep because it wasn't receiving any requests. In that case, it would be woken up again as soon as a request came in for it. If the request in question was a GET, then it would start up normally, but if it was a POST, it would return with a 503 status code and that specific request not be processed -- though the site would be woken up properly. Future requests (both GETs and POSTs) would be handled normally.
We fixed that with this morning's system update, so it shouldn't happen in future -- now both GET requests and POST requests will wake up the website and be handled normally.