Looks like an interesting idea, but even more like utopia at this phase. I knew about it only today, so the answer coming from miniBB team is: no, we have not tried to implement it.
On small forums there is an important question of "personal identity". I suppose with the global thing like OpenID it won't be possible to have a simple username "Paul". I will need to register Paul882128193 or the bullshit like this. Imagine that there will be another few millions of Pauls - Paul882993012, Paul433232929, Paul32123003 etc... Now imagine all Pauls are posting in the same thread on forums.
Is it "identity"? I think no...
Additionally, for miniBB it is not needed to have username at all - you can post in Guest mode in no problem. Of course until it's not restricted by forum administrator.
More... in practice it is working like a nightmare, because I still need to register on each site where I am "logging in" with open ID. As an example: I have registered an OpenID at one of those providers, claimid.com. Now I have to check my email to confirm the registration, and as you might know, about 40% of automated emails are now accessible in spam/bulk folders. So I need to dig my bulk box to search for that email. Finally I am confirming my registration and trying to test on other sites how it works, just entering openid.org's directory of sites and trying to "login and vote" for one of them.
These are the steps necessary for "login":
1) It asks for my "open id" which appears to be a simple URL of http://claimid.com/minibb. I am entering this URL, and the script redirects to claimid.com, keeping the referenced URL (simple at this stage, huh?)
2) There at claimid.com I need to enter my original username and password which were registered at this provider.
3) claimid.com shows "the following registration data was requested" and I need to enter again my Nickname, Fullname, Email - I suppose this data will be passed back to openid.net, and basically, each site supporting this thing, may ask for it - so I am basically registering twice anyway.
Additionally it says "Above changes will only be sent with this response. Edit your profile to make permanent changes". It means I can fill this data in my profile on claimid.com and it will be substituted automatically for the site which asks for it.
4) I am glad I can skip this information and just "login", finally. It already took me few minutes to drive in. Now I am appearing to be "logged in as claimid.com/minibb". What else? Nothing except my "authority" showing I am just logged in.
The idea behind appears simple yet not really effective: just to connect to the 3rd party server, ask for my original login and pass the information that this login was correct. This information most likely then is saved in the original site's session, and when I close my browser, I need to go thought all steps again. Not sure on the "right" sites it is working the same way... with miniBB for example, I can stay logged in for months.
I've tried another site and the was impossible for me to log in because it said "ERROR CODE: OPENID_NOSERVERSFOUND. ERROR DESCRIPTION: Cannot find OpenID Server TAG on Identity page". I have no idea what this means and where should I ask for support - on claimid.com or this other site. This naturally means my "open ID" doesn't work automatically on all sites even if I was logged in already.
I don't see much sense of it then? I need to enter my login on each site separately, thus in 2 or 3 steps. If something doesn't work I don't know what the reason is.
On a regular site it would took me just few seconds. On any bulky site which requires registration, I have only couple of unique usernames to register, and if one of them is already taken, another fits good. I also have one password for such sites which I always remember.
This makes the life easy without such difficult to end user technology as Open ID.