Valve co-founder Gabe Newell has made it clear in the past several months how he feels about Windows 8. He has said that it will be "a catastrophe for everyone in the PC space" and that its users will "basically rage quit computing after they use it." So, it's highly unlikely that we will see any official Steam Windows 8 app released from Valve anytime soon
However, an enterprising app maker has come up with a way for Steam owners to quickly access the games they have downloaded via the Windows 8 Start screen. Developer Jacob Korsgaard recently launched Pin Steam on the Windows Store. The $1.49 app offers a way for Windows 8 owners to quickly pin their Steam purchased games on the Start screen so they can launch them without having to go to the desktop UI.
The Pin Steam app also comes in a free trial version that lets users browse through their Steam library via the "Modern" UI. The paid version is where users will get the Start screen pin support added.
The app developer does point out that Pin Steam requires that the Steam client from Valve be installed. The app also requires the user to set up a custom URL in their Steam profile. Finally, this app is for Windows 8 only. Since Steam itself can't run on Windows RT, Pin Steam will likely be a Windows 8 exclusive for a long time.