There requirements are:
* Distribute under an OSI-approved license.
* Have at least 200,000 public internet users and 5,000 downloads per month.
* Implement OpenID 2.0 support as a Relying Party (RP) or Identity Provider (IdP).
* Pass the OpenID compliance testing tool (to be released in August 2006).
* Make enabling OpenID no more difficult than changing one configuration setting (there can be more optional configuration settings).
* Distribute the implementation as part of the project's core (it cannot be an additional download or patch).
* Place an OpenID logo in the signon form (as on this site).
* Answer "What is OpenID?" (or link to an answer) near the signon form.
I think the 2 requirement is hard to meet for any piece of software. But IMHO PostNuke is popular enough.
As you know Postnuke .8 includes a new login system which is open to all kinds of authentications. It should be a piece of cake to implement the OpenID-system. So if you are interested in doing the job, you should get into contact with the OpenID-people.
Moreover PostNuke could use some pro comments on the OpenID page.
OpenID Code Bounty