Google’s head of search spam, Matt Cutts, posted a video today explaining the difference in how Google handles 404 and 410 error status codes.
Both 404 and 410s are errors for a web page or document not being available, however a 410 is defined as “gone” forever. So 410s are more of an explicit response from the server telling you a page is really gone.
Google handles them slightly differently. With a 404, and some of the other 4xx status codes, Google will “protect” the page and not mark it as removed for about 24 hours. With a 410 status code, Google will mark the page as gone as soon as the error is noticed by GoogleBot.
Matt added, although that is the case, GoogleBot will go back and check both 404 and 410 responses later to ensure the page is really not there. Google is aware of bugs and server issues that happen and thus will check back later, several times, over the course of years, to check to see if the page is ever brought back.
In short, 410s are stamped as removed immediately and 404s aren’t for 24 hours, so GoogleBot can recheck before marking the document as removed.
Here is the video: