I found the following in the Visual Studio 2012 and MSDN Licensing White Paper (emphasis mine):
"A number of free development tools are also available, including Visual Studio Express 2012 for Windows 8, Visual Studio Express 2012 for Web, Visual Studio Express 2012 for Windows Desktop, and Visual Studio Express 2012 for Windows Phone 8. These tools provide a subset of the functionality available in Visual Studio Professional 2012 and are specific to writing applications targeting these platforms. Each of these Visual Studio Express 2012 products is licensed per user and subject to the use terms included with the product. Visual Studio Express can be used to build production applications."
