A recent discussion over in Brad's journal highlighted a common misconception about OpenID: that OpenID users are somehow “less trustworthy” than a site's locally-registered users. While it's true that you can create an implementation in which OpenID users are “less trusted”, there's no reason why they can't be first-class citizens in your system.
It's all down to how your application reacts when it is first introduced to a previously-unknown identifier. You can ask the user to enter any details you like, and validate an email address, and perform a CAPTCHA test, and present a Terms of Service checkbox and anything else you'd normally do when creating a “local” account. It's entirely up to you and your application. Taking things to the logical extreme, you can present the user with a replica of your normal sign-up form but with the options to choose a username and password removed.
Whatever you do, don't go copying LiveJournal's implementation. LiveJournal was one of the first sites to allow OpenID logins, and the community has got a lot of implementation experience in the mean time; LiveJournal doesn't currently follow the vast majority of the best practices that have come about since then. Hopefully at some point LiveJournal's implementation can be improved.
