That's a good question - we removed the old IAuthenticationProvider but never documented how to upgrade.
The IAuthenticationManager/IAuthenticationProvider was intended to provide a sort of single sign-on to applications. It was actually deprecated in version 6.1.6 in favor of the new Authenticator class. If you're interested, the Business Application templates in current releases of DevForce 2010 show how to use its AuthenticationManager with the Authenticator.
In DevForce 2012, single sign-on is provided by default if you login via the Authenticator and set its DefaultAuthenticationContext.