Ah. That's handy. I think it might make the navstrip a little crowded to put the link in there, though. How about where it normally says "Post a new comment" or "Read all comments", between the entry and the comments?

Incidentally, that would also be a handy place to put an "Expand all comments" link, which would be even more useful for me than flat view, because flat view appears to lose the threading.

Either way, I think flat view should be in a FAQ, because I'd never heard of it before.