Microsoft will restrict general distribution of Metro apps to the Windows Store, but grant exceptions to enterprises and developers, allowing them to side-load applications onto Windows 8 devices. While Windows 8 will be an operating system for both desktops and tablets, Microsoft is creating two sets of rules for traditional desktop apps and Metro-style apps, which are optimized for touch screens but will run on any Windows 8 device.

A primer for Windows developers on Microsoft’s website states that distribution of traditional desktop applications will proceed as usual. “Open distribution: retail stores, web, private networks, individual sharing, and so on” will be allowed, Microsoft says. Metro apps, on the other hand, will be “Distributed through the Windows Store. Apps must pass certification so that users download and try apps with confidence in their safety and privacy. Side-loading is available for enterprises and developers.”

This approach is similar to the one taken by Apple with its iPhone and iPad App Store, and also similar to Microsoft’s own Windows Phone 7 Marketplace, although jailbreaks and workarounds allowing side-loading have been released by independent developers for both iOS and WP7. With Google’s Android, by contrast, it is easy for any user to install non-market applications from either third-party app stores such as Amazon’s or by downloading software directly from an app maker’s website. The exceptions carved out by Microsoft will let developers test apps and businesses distribute custom or private apps to employees.

Windows Phone 7 uses a 70/30 revenue split in which Microsoft keeps 30 percent of app payments, and a similar split seems likely for Windows 8 Metro apps. According to the IStartedSomething.com blog, Microsoft’s primer for Windows developers briefly confirmed the 70/30 split for Metro apps but later deleted the information. In other news, we learned last week that while Windows 8 devices with ARM processors won’t run apps originally built for Intel-based computers, Microsoft is working on a Metro version of its popular Office software.