In .Net 4.5, System.Math.PI is written as such, in all caps. Microsoft's own Capitalization Conventions state that Pascal casing is to be used for all public members. Also, their conventions on Names of Type Members mention that all public static fields (which PI is, considering that it's a const) should use Pascal casing.
I'm aware that there's sometimes a convention to name constants in all caps, but the capitalization conventions mention nothing about constants. Pi is also not an acronym like IO, so shouldn't it just be System.Math.Pi?