Today at the Google Blog, product manager Dan Crow formally introduced the "unavailable_after" meta tag, which can be used to identify a temporary page with a set shelf life.
In "Robots Exclusion Protocol: now with even more flexibility," Crow explains:
For example, to specify that an HTML page should be removed from the search results after 3pm Eastern Standard Time on 25th August 2007, simply add the following tag to the first section of the page: <META NAME="GOOGLEBOT" CONTENT="unavailable_after: 25-Aug-2007 15:00:00 EST">
Use of this tag will be treated by Google as a removal request, which should take effect the day after the removal date passes.
Crow first spoke publicly about the new tag earlier this month at an Getting Into Google — Best Turnout So Far!">SEMNE event, which SEMNE co-founder Jill Whalen wrote about in the High Rankings Advisor.