Preview at WWDC likely in June, followed by September launch.
Valve to Expand Steam Store Beyond Games on September 5
The Software titles coming to Steam range from creativity to productivity. Many of the launch titles will take advantage of popular Steamworks features, such as easy installation, automatic updating, and the ability to save your work to your personal Steam Cloud space so your files may travel with you.
More Software titles will be added in an ongoing fashion following the September 5th launch, and developers will be welcome to submit Software titles via Steam Greenlight.
Beyond simply serving as a comparable alternative for the Mac App Store, the Steam store could become a convenient distribution point for software that is not compatible with Apple's Mac App Store guidelines. Apple's limitations such as sandboxing requirements have resulted in some applications not being accepted for distribution through the Mac App Store or required that certain features be removed, and Valve's Steam store is likely to be more lenient in those regards.
Hints of an expansion for Steam appeared last month within the Steam app for Android, which briefly showed new categories of software for the store.