Ask HN: Hosted forum with SSO?

I've got a community site that I built in Rails, and there's a forum kind of tacked on. It's really crappy and I hate supporting it, but some of the users really like it. Plus, it's not really core to what we do.

I'd like to just sign up for some kind of hosted solution that I redirect/CNAME a subdomain to. The only sticking point is that I really want single-sign-on (SSO), so that my users don't have to register new accounts or anything. Preferrably, I would just construct a link that has their user id, username, and userpic URL all hashed together into some parameter that I append to a special link. Then, when they go to the forums from my site, they're logged in automatically.

(Note: tenderapp.com does this for a ticketing system, and I like it a lot)

Any YC companies that do this? Anything else people can recommend?