How to uninstall Microsoft Silverlight from Mountain Lion?
I just spent an hour tearing my hair out, just like you. Deleted everything I could find, and still no luck! Fortunately, I found out about pkgutil when trying to research if other apps have similar issues. Run this command:

$ sudo pkgutil --pkgs


If com.microsoft.SilverlightInstaller shows up, it still thinks it is installed. I had to "clean" (kinda) the installation by using the following:

$ sudo pkgutil --repair com.microsoft.SilverlightInstaller


After that, I was then able to cause pkgutil to forget about the installation:

$ sudo pkgutil --forget com.microsoft.SilverlightInstaller


Hope that works for you!