Don't worry - I was surprised too: it looks like Valve will not be making any SteamOS, or Steam Machine exclusive games anytime soon. Valve has confirmed the news, too.
Designer Greg Coomer talked with IGN, where he said: "You won't see an exclusive killer app for SteamOS from us. We're not going to be doing that kind of thing." Valve spokesperson, Anna Sweet, also chimed in: "That would go against our whole philosophy, to launch something that's exclusive to SteamOS or Steam Machines."
Sweet added: "Whenever we talk to third-party partners, we encourage them to put their games in as many places as possible, including not on our platforms. Because we think that customers are everywhere, and they want to put their games wherever customers are."
I guess this is a good thing, as it means that even a system-seller like Half-Life 3 would reach any Steam-based platform, be it Windows, Mac OS X, SteamOS or any of the Steam Machines.