Microsoft was noticeably one of the big-name companies that didn't put their products up in the Mac App Store. However, it seems they've had a change of heart. Their original absence gave Apple a chance to promote their own iWork Suite but Microsoft is starting to see the benefit of having their own apps feature in the Mac App Store.
The company is doing pretty well with its software selling via other vendors like its own website, Apple retail stores and independent dealers. However, they have admitted to not ruling out the option of putting Microsoft Office up in the App Store for OS X.