Microsoft wants external device makers to create for Windows 8. The Windows 8 app store will install these apps automatically when the hardware is connected, and they can have the same sharing features and Metro aesthetics as other third-party programs.

"Automatic installation means that all your customers will get your app," Eugene Lin, Principal Program Manager Lead for Microsoft, told device makers at the company's BUILD conference this week. "This is really your chance to impress them."

But here's the thing: Microsoft will only install one Metro-style app per external device. That forces hardware makers to be judicious about the software that comes with their products, and abstain from including, say, weight loss software with their printers. "We do that because users can see the physical connection between, 'I connected one thing, and I got one thing,'" Lin said. "And ... it's up to the partners to decide what makes their device a good device or not."

Microsoft's also hoping that Metro-style apps will result in more useful external hardware. Device apps will be able to use Windows 8's "Share" function to make their services available to other apps. So for example, a smartphone's PC software could send images or documents to the phone from within any other application.