Clearly, it's hard to ask for advice on an issue that's not fully fleshed out.

It's just that so many webapps today need to have user authentication/profile management, it seems a shame to have to build your own for two reasons: 1) You're wasting your time building something. 2) People trust you with their personal information, I think people are more willing to trust, say, a facebook, then they are joestartup.com. I say this based on my intuition.