Elizabeth Churchill and Ben Gross looked into this and found out that people find it easier to remember passwords than usernames, because they use the same passwords everywhere, and they end up with multiple different email accounts to handle the problem of having handed them to to all these sites and getting spammed by them.
Meanwhile, over here in the blog world, we've been using blog URLs to refer to people for years, and social network sites have proliferated URLs that are people. I have several that refer to me, my events, my music, my twitters and my photographs linked from the sidebar here. We even have XFN's rel="me" to connect them together, and OpenID to allow them to be used as logins elsewhere, instead of emails.
The underlying thing that is wrong with an email address is that it's affordance is backwards - it enables people who have it to send things to you, but there's no reliable way to know that a message is from you. Conversely, URLs have the opposite default affordance- people can go look at them and see what you have said about yourself, and computers can go and visit them and discover other ways to interact with what you have published, or ask you permission for more.
So, developers, remember that URLs are people too.
Update: This tension between email-as-identifier and email-as-way-to-be-spammed is what makes Scoble's attempt to extract 5,000 people's emails from Facebook for his own use less defensible than it appears at first. Dare Obasanjo recognises the tensions, but strangely dismisses the OpenSocial attempt to abstract out this kind of data into a common API.