Cannot step into XML Serialize() method in VS2010?

Question

I'm trying to get to the code behind the Serialize() method in the .NET framework but despite having followed all the steps on the Hanselblog (and the ones that he references) it's not happening for me.

I'm running VS2010 Ultimate on x64 Win7 and would appreciate any help that anyone can give.

EDIT: 2010-08-31 I flattened and rebuilt my machine just in case there were some traces of beta software on it but it stubbornly refuses to go into the Serialize() method. It goes into some source code files but not the one I need.

Any ideas?

Derek