Google are just testing an approach called "federated login" which they think is the usability solution to the so called "URL problem" which is that Joe Average doesn't know what a URL is or how it should be formatted let alone what his or her URL is. Obviously MySpace users do, but there is also an implication from Google that all users are comfortable using an email address. Many of the younger demographic use FB messaging or YIM for communication and only use their email for "official things".
There was a lot of discussion about all these issue at http://therealmccrea.com/2008/10/20/live-blogging-the-openid... and all the interested parties want to resolve this. I should know I spent the day with them there.