Best structure for centralized User DB over multiple membership-driven sites?

Question

We've built a social networking site for a client. It did very well and now they want to package it up and license multiple copies of the same site but branded for their client.

Each site is fairly autonomous except that users on one site can access the content from users on another site, requiring that user profiles be centralized.

We have an idea of how we're going to set this up but what would you do to make it functional and scalable?

Answer 1

Perhaps look into OpenID like SO has. The only thing you'd need to do is convert the existing site, but then the users will have to re-register ... or ... you could offer OpenID on the original site as a secondary login method for those users who want cross-site membership.