Tim Bray with a great analogy:
I quote from The Users API on the Google App Engine site:
Google App Engine features integration with Google Accounts: Applications can have users sign in using their Google accounts, and know who is using the application during a session. With Google Accounts, your users can get into your application sooner by not having to create a new account, and your application can personalize the user experience without managing its own login system.
... and now you’re a sharecropper on Massa Google’s plantation.
Very smart move on Google's side. While this is probably perfectly fine for small sites that don't have any larger goals, it's obviously unsuitable for those who want to build a new web business with the goal of selling it to Google later. The question is: will people know which group they belong to?