Google's aiming for the social game, and has been hosting, indexing, and correlating it for a long time. They have blogger, buzz, gmail, people's names, credit cards, links to and searching of your twitter account, your blog, your friends, your x, your y, even your z. Why can't they infer popularity that way, identically to how PageRank works?

And people do see who +1d something, it requires a public account - it's visible on the link (as displayed in the video, and implied by the same behavior with tweeted links) and on your profile page.

If a nobody from nowhere +1s a link, their friends will see it, but it's unlikely to affect the world at-large. Similarly, trust-metric wise, if a cluster of nobodies +1s a thousand pages, the impact will be restricted to their cluster. If an extremely-highly-connected person, who many people follow on Twitter / Reader / etc, +1s something, it'll have more impact simply by being visible to more people and because their "importance" can be inferred by those connections.