Someone, let's say Yahoo, could roll out their own twitter-like service (let's call it yatter). And all yahoo mail users "become" yatter users automatically, like how they became Openid accounts.
- when a yatter user "joe" yatts, "@twitter/charlie what are you doing?"
- yatter's twitter bot called "yatter" relays the tweet, "yatter/joe: @charlie what are you doing?"
- twitter user "charlie" sees the message in his @replies stream, and can respond, "@yatter/joe hitting my refresh button again and again"
- a yatter.com bot called "twitter" relays the message, "twitter/charlie: @joe hitting my refresh button again and again"
- yatter user "joe" sees that in his @replies stream in yatter.com.. (repeat from the top)
I'd guess it makes sense for yatter apis to be fully compatible with twitter.com, in the ma.gnolia.com vs del.icio.us kind of way. So twitter clients just need to add a new config, "hostname", and we're done.
I think yatter.com could technically allow @yatter/joe to follow @twitter/charlie. The question is, would twitter.com allow their @charlie user to follow @yatter/joe?
Yatter could use the age-old hotmail trick and append the latest status message of the mail sender to the email footer. That should pique interests of recipients. Besides, "a signature" is an existing, understood concept that users can easily see why they'd would want to update their "yatter status"..