A thread at Search Engine Watch forums named Expired HTML Pages beating me out discusses one member, being upset that documents that are represented as being expired as ranking above his site. There is an expires meta tag that looks somewhat like <META HTTP-EQUIV="expires" CONTENT="Wed, 26 Feb 1997 08:21:57 GMT">
So should Google and other engines use this data to determine if a page should not be in the top results? As Phil Craven points out in the thread, "I've never heard of Google dropping pages because of a meta "expires" tag. They do use "last modified" information to speed up crawling, but not when it's in the page itself."