Heh, well, I'm pretty sure Eric isn't talking about enabling this for the articles, which I REALLY would think is a bad idea, and I'd have to raise more hell about it for a long time.
He's only talking about the User pages and the Talk pages, right?
That's correct, however, you will still get a backlink to
[[Talk:GNU]]
on
[[Talk:GNU/Linux]].
Without breaking compatibility with existing subpages, the software can't know whether Talk:GNU/Linux is a subpage of Talk:GNU or a separate Talk page.
This is a very rare case, though; I can't think of any other one. If you absoutely want to avoid that single backlink, disable subpages for Talk pages (we can still allow them for User talk pages); however, they are very handy for archiving stuff.
Regards,
Erik