Is there a HTTP status code to tell Google (and others) to go away, index me again later?
Basically, one that semantically tells clients that the site is down for maintenance?
The ones I have considered are
304 => Not modified
307 => Temporary redirect
410 => Gone
503 => Service Unavailable
I'm leaning towards the last one, but was just curious as to which one was proper choice.
HTTP 503 - Service Unavailable would be the most appropriate.
The Web server (running the Web site) is currently unable to handle the HTTP request due to a temporary overloading or maintenance of the server. The implication is that this is a temporary condition which will be alleviated after some delay.
This post on the Google Webmaster Central Forum is also relevant:
Yes - a 503 is the correct server response for "We're closed". If you substitute a normal HTML page saying "We're closed" and serve a 200 it's very likely to get indexed by Google.
If you give the Googlebot a 503, it will just go away and come back later without indexing what you give it.
UPDATE:
As for the php implementation, you may be interested in checking out the following article:
Also notice the retry-after header, where you can specify after how many seconds another request should be re-attempted.
From http://www.google.com/support/webmasters/bin/answer.py?hl=en&answer=40132
503 (Service unavailable) The server is currently unavailable (because it is overloaded or down for maintenance). Generally, this is a temporary state.
I'd consider the 503 to be most appropriate, since the 5xx-group is meant for server errors in general.
