Sadly, the comment facility on the site seems down at the moment, so here's the response I would have posted there:
You're dead right - it could replace some aspects of the email system. It could also replace some aspects of the instant messaging system.
I've also been thinking along these lines - peer-to-peer networks have shown the way for file exchange, FOAF/RSS et al could easily do the same for any kind of inter-person information exchange.
Underlying it all is the change in the way Internet connectivity is being provided. When it was via expensive dialup, then the mode was to connect, grab your mail from your ISP's mailserver and send any outbound mail to it for batch-delivery.
Now that always-on connections are more commonplace, there's less need for that any more - if I'm online the whole time I can run an RSS feed instead, and people who used to get email from me can now poll my RSS server for any new messages I might have to give them.
Naturally, this requires a means of authentication and a flexible means of access control at my end - so roll on the day when aggregators support *any* means of supplying credentials to the other end (https, basic/digest auth, cookies, X.509, PGP, prompting for the answer to a question, credit card payment or whatever else the server owner deems appropriate) - this is XML after all, so no barrier exists to having a <auth:credentials>...</auth:credentials> concept.
Death of email predicted... RSS at 11 :)