I don't know that it's going to happen anytime soon, but I can see exchanges between karma and real currency eventually leading to a reputational economy. See Doctorow's Down and Out in the Magic Kingdom. http://craphound.com/down/download.php I wish that pg would start allowing karma transfers here so we could start experimenting and I could cash in all of my stupid points, but that experiment is not one of his goals.
For a while I've told myself that I'd like to auction this username for charity, mostly just to see what happens, though I like the idea of converting my years of activity here into a charitable donation.
On Hacker News, karma really does just measure activity. My point is that it is theoretically possible to give karma more meaning than it has today.