Q: Is it possible to limit login to some users, like htaccess/htpasswd does?
A: No. It is possible to limit authentication to certain identity providers (by using AuthOpenIDDistrusted and AuthOpenIDTrusted, see the main page for more info). If you want to restrict to specific users that span multiple identity providers, then OpenID probably isn’t the authentication method you want. Note that you can always do whatever vetting you want using the REMOTE_USER CGI environment variable after a user authenticates.
Funny, this is just what I thought was most interesting about OpenID: it lets you build sites where you can offer a varying experiences (including letting them in or not) to differ users based on what you know about them. OpenID itself doesn’t do everything out of the box, but by associating public URIs with people, it’s a very useful step.