Thanks for the feedback. We're still working on nailing the design - we need a way that all the buttons look uniform, and want a way to explain what they are (namely Persona, as I said, still working on it.) And fitting the multiple options in the small space is no easy feat. Fortunately hover isn't necessary for interaction, so the touch screen device point is moot. And the "moving target" is the same reaction as people gave Apple's dock. But you're right - this design isn't perfect yet.

Once a user's source authenticity is verified, we give the option to merge accounts once we detect overlaps. So if you login via Twitter, verify your Yahoo! email address through the profile, and then login through Yahoo!, you'll be in the same account as you were in the 1st login. We have ~20 scenarios that we maintain for logins, and have heard 0 complaints on the assumptions we have in practice.

Thanks for the screenshot - we'll get that fixed today. Would you like an email once it is resolved?