This came on my radar last year. It's a page from the IBM research site with ideas for their next gen email features. Me and a friend were talking about this today and I figured I'd post it when I finally found the page. The ideas range from really nifty and new (thread arcs) to "why the hell didn't I think of that (inbox division with date separators).
It'd be nice to see some of these features (or even just the obvious and non-patented ones) implemented in some of the newer email clients (ie: Evolution).