I am both interested and worried about OpenID. The interest is personal, and through the OLPC project, so I'll be happy to help get things in shape for inclusion in Moodle proper...
The concerns I have come from a review ofthe protocol. When I read it in detail, I found that it is wide open to "phishing" and man-in-the-middle attacks, and that it trains users to let unknown websites "redirect you" to "your trusted" website, where they'll put their usernames and passwords. Sounded like a scammer's wet dream.
Wondering whether I was crazy, mentioned it in the OLPC dev list, and Ben Laurie (one of the key developers of
Apache's SSL support) agreed with me. In fact, he's been pointing out how naive the protocol is for a while. He's not the only one either.
The only safe and sane solution for the kind of federated identity/SSO that OpenID wants to provide (and it's a very worthwhile thing to aim for!) is to modify webbrowsers for this. The trust cannot be established or checked with just HTTP and webpages. Luckily, the OpenID gang have heard the many voices and seem to be working towards that.
In the meantime, purely HTTP based OpenID with vanilla browsers is not something I would recommend to anyone who cares about security. Instead I'd put a warning sign with BLINK tags around it.
(OLPC will have OpenID integrated in the webbrowser and OS, in case you are wondering

)
Ben's posts and following discussions can be found here:
http://www.google.co.nz/search?q=ben+laurie+openid
And some more links
http://marcoslot.net/apps/openid/
http://www.idcorner.org/?p=161
http://www.google.co.nz/search?q=phishing+man-in-the-middle+openid&btnG=Search&meta=