I think of reputation as 'aggregate opinion'. Someone's individual opinion of me, while potentially feeding into my reputation (if they tell others), is not reputation itself. That's why, for me, some IDP asserting to some attribute of mine is not attesting to my reputation, unless that IDP was a Reputation Provider (ala Opinity) specifically doing so.
But, opinion aggregated from which community? Or, in other words, what set of other users are asked the question 'What do you think of Paul?'.
In eBay's feedback system or Slashdot's Karma, the community is comprised of all other users with whom I've interacted (within the respective systems). For those other eBay users scoring me, and for those relying on such scores, I am my eBay pseudonym. Neither group knows me beyond that identifier, nor do they need to, because the whole scope of the system (interactions, subsequent scoring, and querying of reputation) is confined to the online world, specifically eBay's corner of it. The only point at which the online and physical world intersect is at the point of shipping whatever product was bought and sold.
Another potential community of users from which my reputation could be built are those that do indeed know the real physical me - my offline friends, family, and colleagues. Such a group will almost certainly have insights about me unavailable to soem eBay user who knows me based only on our online commercial transactions.
So, my social network, generally comprised, at least in the first degree, of individuals who know the real me, and not some online pseudonym by which I present myself, could be a basis for reputation. I think it would be interesting to explore when this sort of offline/online bridging reputation would be more applicable than pure offline.
As one example, when someone is considering hiring me, they won't be thinking to themself 'I wonder what his eBay Feedback score is'.