PRO: It seems that some services are considering making e-mail the standard for identifying a specific user across the net:
e.g. http://www.techcrunch.com/2009/08/14/google-points-at-webfinger-your-gmail-address-could-soon-be-your-id/
CON:
This hasn't happened yet, and there are lots of other options such as OpenAuth and OpenID that are around now and have some support (you also have login using Facebook spreading).
Just tailor your choice for identity to whatever the target audience of your app will be.
