In a video released by Google's Matt Cutts yesterday, he described the technical difference between a typical 404/4xx code and the 410.
- 410s GoogleBot will mark as gone immediately upon discovering it.
- 404s or other 4xx GoogleBot will "protect" for 24 hours, try again, before marking it as not found.
In both cases, Google says they will retry the 404s and 410s in the future. Just in case there were server or coding issues on the site. But with 410s, they get marked as gone right away.
Here is the video:
Forum discussion at Google+.
Note: I am offline today and this post was pre-scheduled to be automatically posted. So if I am delayed in responding to issues or comments, I apologize in advance.