A Google Groups thread has some details that I have not seen before. If a site is temporarily down, I thought Google would not deindex the site, but give it some more time before they removed the page from the index.
According to this thread, that may not be the case.
I've been having problems with downtime from my hosting company (am moving tomorrow as this is the final straw) and I've now practically disappeated from Google. I was in 3rd position for 5 of the more popular searches for my business and now - nothing.
Vanessa Fox of Google replies:
If the host is down when Googlebot tries to access your pages, then those pages may disappear from the index until Googlebot can crawl them again. In webmaster tools, do the pages you want indexed appear in the crawl errors section? If so, then Googlebot was unable to access them.
If you are moving the site to a new host and the pages are available the next time Googlebot tries to access them, then you should see them in the index again soon after that.
So Google may remove the pages from the index if your pages are not accessible to Google due to server issues.
But she explains a reinclusion request is not required in this case.
You're right that requesting reinclusion is only for sites that have violated the guidelines. This isn't the situation in your case, and there's no need to contact us to let us know that your site has moved and is available again, as Googlebot will keep rechecking for it automatically.
I have added some questions to the thread for more clarification.
Forum discussion at Google Groups.
Update: Besides for the excellent comments below, Vanessa replied to my questions saying:
Googlebot will try a few times before the pages drop from the index.
As for how long it takes for a page to get back in once the site is back up, that really depends on a number of factors, such as how often the site is crawled in general.