Run Windows Store Apps in a Window on the Desktop in Windows 8?

First install Visual Studio Express 2012 for Windows 8, then you can use the Windows Simulator app to run any Metro/Modern/Microsoft-style UI app in a window:

This is how developers test their apps. You can also use pinch and rotate gestures in Windows Simulator.

Windows Simulator connects to your PC through RDP, so, all the apps including Desktop apps that are currently installed on your PC can be used in the Windows Simulator.

You can open Windows Simulator from here if you have 64-bit/x64 Windows installed:

 %ProgramFiles(x86)%\Common Files\Microsoft Shared\Windows Simulator\11.0\Microsoft.Windows.Simulator.exe

and from here if you have 32-bit/x86 Windows installed:

 %ProgramFiles%\Common Files\Microsoft Shared\Windows Simulator\11.0\Microsoft.Windows.Simulator.exe

Is this possible? I'm imagining developers needing to view/run/build their apps in a desktop window in order to work productively. Is this the case?

If the application has support for the snapped view, you can snap it to the side and run the desktop app as usual. Else there is no way.

No, this isn't possible. The Metro apps are run in a sort of sandboxed way and cannot be run in your desktop. Paul Thurrott has two pieces on this where he states that Microsoft chose to not allow this because

Metro-style apps and the desktop needed to be kept separate in order to retain Metro’s built-in security functionality.

EDIT: more extensive info on this design decision can be found in Paul's article 'Windows 8 Consumer Preview: Why Metro and the Desktop Don't Mix'. In short it comes down to

• focus on mobile devices
• battery life
• security (the sandbox model)

There is a 3rd party tool called ModernMix which allows you to run Windows Store Apps in a Window on the Desktop:

with Windows 8 and 8.1.