Barry on Search Engine Roundtable has written a post on Does Google Still Crawl 404 Not Found Pages?
A WebmasterWorld query raises the question, “how long will Google ask for a page when it gets a 404 (Not found)?”
A reply to the post states:
They check that they are still gone, almost forever.
They do that just in case one day, they are no longer gone, but instead are republished.
Say you bought a domain name from someone and put up a new site. Say that Google refused to pick up your /about.html and /contact.html pages, and that eventually you found that it was all because once a page went 404 Google refused to ever look at that URL again.
You would think that was a bad policy. That is why it doesn't work like that.