How to protect C# program from .Net Decompiler ?!

Hi,
As you know there is some .Net Decompiler as Reflector that allows getting .Net DLL Source Code!

Do you think it’s a real threat for all .Net Programs?! Or maybe the first Microsoft steps in the Open source world? Or a bad news for Microsoft because they did not expect that?

I found on the web some tools to protect C# programs…but they could affect the performance of the original program.