Following up on my focus on making this a very robust *core* for user/group stuff, I've spent the last week putting in place a system if "bridged" logins.
This means users will be able to login to a yumps site using openid/facebook/twitter/etc, with very robust support for managing their linked/bridged logins (creating multiple ways to login, removing them, optionally requiring additional security like local password), and automatically creating local accounts from the bridged logins, so that the entire process is transparent to the rest of the site -- including all the normal operations for validating/approving accounts, etc.
In other words, if you build a site using yumps -- your users should be able to create accounts and login normally, or using a wide variety of 3rd party sites, and it should happen totally transparently to you as the site coder. Yumps will also be a host (via something like OpenId) for providing such authorization of yumps accounts to other sites.

