The example on http://msdn.microsoft.com/en-us/library/ms164307.aspx implies that files can conditionally be included depending on the currently selected configuration.
This does not work as described, though.
The attached project will always include the file "AnyCPU.cs", even if the target platform is changed to x86 or x64. The reason is apparently, that the $(Platform) property gets its default value at the top of the project file and it is only updated to its actual value as selected by the solution configuration after the entire project file has been parsed.
Since conditions on propertygroups (e.g. in line 12 and 13) do work as expected, I consider this a bug.
IMHO, the following construct should also work, but currently doesn't:
<Compile Include="$(Platform).cs" /> <<< should include x64.cs or x86.cs but not AnyCPU.cs