In October, we reported that Google treats a 410 stronger than a 404 status code. I am not sure exactly when Google made the 410 status code that important, but I believe it was sometime in October.
A new thread surfaced in Google Webmaster Help where it was suggested by a Googler to use the 410, once again.
Googler, Jonathan Simon said:
Last year we started started treating 410 Gone responses as being a stronger signal for indicating when a page no longer exists. So for the situation you describe (where 301 redirects aren't practical) returning a 410 response is the way to go.
It seems to me that when Googlebot discovers a 410, it will communicate to Google to not index that page for a really long time. I believe Google will check pages they know to be 404ed every now and then. Maybe a 410 will tell Googlebot to never check back or check back extremely rarely?
Just keep in mind that the 410 is much stronger than a 404 response - it may come in handy in your future SEO projects.
Forum discussion at Google Webmaster Help.