This is a great feature but is older than GMail. Lotus Notes mail for instance allowed the same mail to appear in different views, including an "all documents" view that had, as you might expect, everything in it. This advance was rather let down by the rest of the Notes client interface however: http://hallofshame.gp.co.at/lotus.htm
Wow, blast from the past. My first serious use of email was Lotus Notes. It had some really cool features but also really strange ones, such as using the escape key to send a message.
