http://googleblog.blogspot.com/2007/07/robots-exclusion-protocol-now-with-even.html

META NAME="GOOGLEBOT" CONTENT="unavailable_after: 25-Aug-2007 15:00:00 EST

The date and time is specified in the RFC 850 format.

This information is treated as a removal request: it will take about a day after the removal date passes for the page to disappear from the search results. We currently only support unavailable_after for Google web search results.

After the removal, the page stops showing in Google search results but it is not removed from our system. If you need a page to be excised from our systems completely, including any internal copies we might have, you should use the existing URL removal tool which you can read about on our Webmaster Central blog.

Meta tags everywhere
The REP META tags give you useful control over how each webpage on your site is indexed. But it only works for HTML pages. How can you control access to other types of documents, such as Adobe PDF files, video and audio files and other types? Well, now the same flexibility for specifying per-URL tags is available for all other files type.

We've extended our support for META tags so they can now be associated with any file. Simply add any supported META tag to a new X-Robots-Tag directive in the HTTP Header used to serve the file. Here are some illustrative examples:

   * Don't display a cache link or snippet for this item in the Google search results:

X-Robots-Tag: noarchive, nosnippet

   * Don't include this document in the Google search results:

X-Robots-Tag: noindex

   * Tell us that a document will be unavailable after 7th July 2007, 4:30pm GMT:

X-Robots-Tag: unavailable_after: 7 Jul 2007 16:30:00 GMT

You can combine multiple directives in the same document. For example:

   * Do not show a cached link for this document, and remove it from the index after 23rd July 2007, 3pm PST:

X-Robots-Tag: noarchive
X-Robots-Tag: unavailable_after: 23 Jul 2007 15:00:00 PST