Have they backpedaled at all on their decision to not build executables for XP? I couldn't find any mention on the Jason Zander blog, so I assume not.
The C++ Runtime Library doesn't support XP, so C++ programs compiled with Visual Studio 2012 can't run on Windows XP. (It is possible to work around this by statically linking and overriding some CRT functions, as detailed here: http://tedwvc.wordpress.com/)
You can keep using the Visual Studio 2010 C++ compiler (and target XP), but then you can't use any of the new C++11 features supported in VS2012.
.NET Framework 4.5 RC also doesn't support XP (see "System requirements" at http://www.microsoft.com/en-us/download/details.aspx?id=2990...), so C#/VB.NET programs targeting .NET 4.5 can't be run on XP either.