When you put a Drupal site in maintenance mode, non-administrators see the standard maintenance mode page (assuming you clear caches after doing so). If you examine the response, you will see that it is sent back with a HTTP status code of 503, which from RFC 2616 is:
503 Service Unavailable
The server is currently unable to handle the 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. If known, the length of the delay MAY be indicated in a Retry-After header. If no Retry-After is given, the client SHOULD handle the response as it would for a 500 response.
And from the Official Google Webmaster blog:
If my site is down for maintenance, how can I tell Googlebot to come
back later rather than to index the "down for maintenance" page?
should configure your server to return a status of 503 (network
unavailable) rather than 200 (successful). That lets Googlebot know to
try the pages again later.
So, that gives evidence that Drupal does the right thing, and that Google will revisit your site and index pages the next time it gets back a non 5XX status code.