Would it be difficult to add an OpenID provider to PM?
No. It would be semi-trivial. Probably something like 50-100 lines of code + Net::OpenID::Server.
A good idea?
I think "no." It sort of locks PM into being an identity provider. That's kind of a big commitment for little, if any, gain. Mostly it would be the programming equivalent of a vanity plate. There are several good/free OpenID providers out there already. If you want to do your own from your own site, it should be pretty easy to roll (after you get your head around the protocol -- it's confusing at first). If I could choose, I'd prefer OpenID to generally reflect 1:1 ownership of domains or accounts at dedicated providers, rather than participation in social (even code based like PM) sites.