This is how Staticloud[1] works. You put in your email address and receive a log in link. You never have to register; registration and login are the same process.
Very interesting idea! Although I'm not sure that it would actually make it easier for users that don't do tabbed navigation (mom) to log from a second/public computer.
So, if elegant for power users, and 'not easier' for mom-users, should we implement it?
I agree that the UX isn't perfect, but neither is our current login/password system for every site. For me personally, if I don't use a site regularly my login process turns into: click forgot password link, check my email, reset the password, then login. Then again the next time I visit the site 3 months later. So if that's what we're going up against, the email to login session flow isn't too bad.
[1] http://staticloud.com/