I was also thinking about something similar a while back until I discovered oStatus (http://www.ostatus.org) and its related projects (ActivityStreams, PubSubHubbub, Salmon, etc). While I think an implementation over SMTP could work quite well, I think using PubSubHubbub has a reasonable advantage in that doesn't require app developers to use ports other than standard HTTP ports (80 & 443). This isn't a big deal for major websites, but part of the advantage of having a truly distributed social network is that even the little guys can join the game--which means it should be as easy as setting up something like say, WordPress.