It seems that sites with considerable downtime can be deindexed by Google. In fact, a member at Google Groups has reported that his site has vanished from a previously third position for many searches. The reason could be downtime.
Vanessa Fox has responded by saying:
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.
But that is not a definite assurance that sites will not be removed if they were down for server reasons. The Google crawler keeps coming back to the site and if your site happens to be down most of the time, it might as well go off the index. Vanessa has also explained that reinclusion is not necessary in such cases as the site will be indexed automatically.