Or, if the Github team, having first created a GUI for Mac, which they use themselves and can more easily test, were to then create a Windows version and charge $0 for it, they could convince a lot of Windows Subversion users to switch to Git and Github, paying monthly for private repos.
Which looks like where they're going with this.
If someone made an app like Tower but for windows and charged $50-100 for corporate licenses, they could make a killing.