I'm so pleased to report that setting up OpenID is way easier than I thought it would be. I used the excellent "getting started with OpenID and PHP" guide from Zend to integrate OpenID with PhunkyBB and Nexista.
My implementation isn't totally done yet, but the OpenID integration is. I have to link openid strings with the phunkybb database, connecting the user_id field with the user openid.
That shouldn't take long, and I'm very excited to get this launched and live. In addition to PhunkyBB, I've also added OpenID support on the Docunext wiki using a mediawiki plugin.