What HTTP status code should an unconfigured domain return?

Solution 1:

Use a boilerplate HTML file and robots.txt - if you may not get around to implementing the domain, it could help to have your contact details posted so potential buyers can reach you (particularly if you use a WHOIS proxy registration).

Solution 2:

404 (Not found) is just fine:

The server has not found anything matching the Request-URI. No indication is given of whether the condition is temporary or permanent. The 410 (Gone) status code SHOULD be used if the server knows, through some internally configurable mechanism, that an old resource is permanently unavailable and has no forwarding address. This status code is commonly used when the server does not wish to reveal exactly why the request has been refused, or when no other response is applicable.

In your case, there's nothing matching the URI in your vhost (yet).

Of course, you can return a page saying whatever you want at the same time, it doesn't have to be the default error page.