> I think Animats is calling it zero sum because the value of Bitcoin is essentially based on speculation rather than use so relies on new entrants to prop it up and the coins are generated within the system.

This really depends on what you think is going to happen going forward. Surely the current value is propped up by speculation, but are the speculators right? Is future-Bitcoin going to be very useful and thus valuable for something, so that the speculators will eventually have someone to sell their Bitcoins to other than just more speculators?

It's not impossible for that to happen just because it hasn't already happened. It's also not impossible that it doesn't, in which case the speculators themselves will be the ones left holding the bag. But that's how it goes, isn't it? High reward, high risk.