> Some sort of centralised system for verified identities online with a simple API that people could plug into their forums/comment systems on blogs.
This would not even be particularly hard; the infrastructure already exists in the form of SSL certificate issuance/verification.
PGP keysigning is supposed to accomplish the same thing but has never seemed to work as well in my experience. It's a lot easier to just pay $20 to Thawte or Verisign and send them copies of some identity documents than to chase down volunteers and get them to verify you. (Not because there's anything wrong with the volunteers, but I always feel like I'm imposing, in a way that I don't with Verisign.)
And if the cost component to SSL/X.509 is problematic, there's always CACert -- they have a fairly rigorous system for identity verification (at least on par with the commercial issuers), using either trusted third parties or a web-of-trust -- and they don't charge for it.
I think there are JS libraries around already for doing S/MIME signing in-browser; you could build that in to a comment form and just reject anything that wasn't signed with a trustworthy certificate bearing a real name (not "Thawte Freemail Member" or something similarly generic).
Of course you'd lose 99.9% of prospective contributors, but that's the case with any barrier to entry that removes anonymity. People aren't going to join something like that unless the content is really good.