Does Valve really needs more than 0 confirmations? double-spending doesn't seems like a big issue for them since Steam is DRM and they have some level of control over the software people have in their libraries (and total control in the case of online games). If the concern is whether some people would just download games, double-spend before getting one confirmation and then keep Steam offline forever (or copy the game files from the steam folder), I think people can already do something similar as they let users "return" games during the firsts 24 hours after the purchase.