Yeah, 410 is for when you know something has been deleted and you know it's not coming back.
The actual 410 code is sent in the HTTP header, so the user never sees it (unless they can check the headers or it appears on an error page). There's a whole bunch of status codes in use that nobody knows about. For example, if you can access a page with no problems, you get 200. If the page has been moved permanently, you'll get a 301. If it's contents hasn't changed since the last time you viewed it, you'll get a 304, and so on.
They act largely in the background, and they're there largely for the software that a user is using, but the software often acts differently when they see certain codes. For instance, IE displays the friendly Not found page on receipt of a 404, and all browsers change the address when they see a 301.
The benefits wouldn't be direct. For instance, search engines might update their indexes more accurately, and as I've mentioned before, visitors requesting deleted posts would know what had happened to them.
Adding a 410 feature to wordpress would be beneficial for the visitors to the blog, not necessarily the owner, and since a fundamental part of blogging is that it gets read, it seems a good idea to make things better for your visitors. But I will concede that it only makes things slightly better :)
It's probably more appropriate to put this in a plugin. Maybe I'll write one when I know a bit more about the WordPress API.
Anyways, I found another enquiry about returning a 410. I'm not completely alone!