I was under the impression before that Valve would have to buy X copies of a game at whatever price the developer sold it to them at, and then sell it themselves for whatever price they thought they could get the best profit (including rate of sales) at. Which, I think, is the same way an actual store would do it.
Still makes sense to me, considering how they seem to over-sell and give out bad keys to some games during these sales, but I don't actually know how they actually do it, so yeah. Are we all speculating or does someone actually have accurate information?
Wild guessing on my part, though none of my steam games actually have CD keys, save Arkham Asylum which uses GFWL (it was a gift). I hadn't taken that into consideration, I just assumed there weren't "copies" per se and you just authenticated on a valve server that checked your login against the list of games you own and cleared you to run it. Digital distribution removes the need for copy protection, so it would be more economical for both parties to just give steam the master data files for the current release to distribute, and charge money to have that game linked to your "authentication approved game list."
Man, it's getting late and I'm babbling. Sorry.