What it is about is not much really. If you never heard about OpenID I recommend the Screencast by Simon Willison as a first introduction.
OpenID is a simple distributed identity system, though identity does not mean verified identity here. It's just a small convenience. Instead of creating a free user account on all the sites you need to register for doing stuff (editing a wiki, posting in a forum, adding a comment in a blog) you can automatically login there with your OpenID. It's not about any trust or additional security - you can create a fake OpenID as simple as registering a fake account on the site it self.
But convenience is good, isn't it? So what about having OpenID logins in DokuWiki? I went through a few OpenID PHP libraries, but they seemed overly complicated for what I had in mind. I didn't need any OpenID server components, I just wanted to login through it. So as usual I wrote my own. The specifications are open but not as simple as they could be in my opinion. Especially the flow of data wasn't 100% clear to me at first.
But finally I understood and just released the first version of the OpenID plugin for DokuWiki. I implemented a very simple OpenID consumer1) within an action plugin. In hope to make it easier for others to implement similar functionality I will outline its workings here.
The above is the simplest method of implementing an OpenID consumer (called dumb mode) there are different ways which rely on some cryptography in the consumer itself. In dumb mode all cryptography is done on provider side.
So the whole thing works, but I haven't installed it at wiki.splitbrain.org, yet. This is because I'm still thinking about the implications this could have. What would be very convenient for users could prove much more convenient for spammers. I'm not the only one who asks this and Simon Willison's answer seems to suggest that I should distrust OpenId users a little bit more than usual users…