I'm not unfamiliar with a Magento 503 error. However, for a client I'm getting this error without any markup to the page as well and the full error reads:
The server is temporarily unable to service your request due to maintenance downtime or capacity problems. Please try again later.
Additionally, a 503 Service Unavailable error was encountered while trying to use an ErrorDocument to handle the request.
Normally I would start looking for the "maintenance.flag" file, empty my cache, deactivate and clear compiler or see if there have been any new extensions installed. I've done all this just to be sure but especially the last sentence gives me the idea this is a problem outside of Magento. I'm also not able to access the backend as this throws the same 503 error.
What I've also done is uploading a ".html" file which is returned normally. Uploading a ".php" to the root is NOT rendered and returns the 503... What does this tell me?
What steps other that described can I take to be more sure?