Getting Into Google by Jill Whalen reports Dan Crow, director of crawl systems at Google, says that Google is going to release a new META tag named "unavailable_after."
This "unavailable_after" tag will allow a webmaster to tell Google when the Googlebot should no longer crawl that page.
Jill explains that this tag comes in handy when you have a limited time offer promotional page, and on this page, the promotion will expire on a specific date. By using the "unavailable_after" tag, you would be able to tell Google that they should not crawl this page, after the promotion expires or after it is no longer valid for any other reason.
I think this really could be helpful in some cases.