Windows 8 is a major problem for Steam on Windows and is going to make a huge dent in Valve's profits. Microsoft's integrated app store (Windows Store?) is going to have an instant userbase and it sounds like Valve expect quite a number of publishers to use it. Especially if it can be used to bring Xbox and PC versions of games closer together.
Users will be in the position of either buying a digital copy of a game directly via windows, or via a third-party client (Steam). Considering that the majority of PC users (NOTE: Not PC gamers) don't know what Steam is, they are going to use the Windows Store rather than faf around downloading and installing another client.
That's why Valve are setting their sights on Linux and praying. They are set to lose a large chuck of money if Windows 8 takes off.