On Twitter, Om Malik flagged this post by a venture capitalist calling email the "killer app" for web communications. It's a great post that you should definitely check out.
What I'd add to this post is that the email address's role as a sort of universal identifier is very similar to the role that your telephone number played in the past, and still does, to a certain extent. There are still contact centers that ask you for your phone number or that use ANI to generate the screen pop that gives the agent your information. If you call your local pizza joint to place an order, chances are they'll ask for your phone number to get information like your address for delivery.
(As an aside, how cool is this? Whatever you think about the politics of what's been going on in Wisconsin, it's been reported that protesters in Egypt ordered pies from a pizza parlor in Madison for the demonstrators there. The real sign of how transformative the Web has become is that we all read that and know immediately that this was really no big deal, in terms of getting it done--of course somebody in Cairo can locate a pizza parlor in Madison, Wisconsin near the state capitol and can order a pizza via PayPal or whatever. They could probably even figure out exactly who to have it delivered to.)
Your telephone number will likely remain voice communication's "connective tissue," in this VC blogger's elegant phrase. The coming evolution and eventual breakdown in email's dominance, which he discusses later in his post, are paralleled by what's happening in voice: If you've cut your landline, you still have a mobile number, but that's probably not helpful as a physical locator, the way it was when area codes always defined geographic spaces where people really were sitting at that moment. (The area code for my new corporate BlackBerry is 331. I had to go to Wikipedia to find out where area code 331 is. Turns out it covers both our office location and my home town. Who knew?)
Telephone numbers also don't matter for Web-based calling like Skype and Google Voice. Then again, your universal locator/identifier for Skype is your email address.
So as the connective tissue of phone numbers atrophies, it's been replaced by email addresses. There probably will always be a need for a common platform that can function as this lowest-common-denominator--though what it'll be after email, I don't know.
