Thanks for the prod! As soon as you said '64bit' it reminded me about the 'Wow6432Node' which all the 32bit keys reside under on a 64bit machine (including mine).
The solution is to follow the steps above, except use the key 'HKEY_LOCAL_MACHINE\\SOFTWARE\\Wow6432Node\\Microsoft\\VisualStudio\\10.0\\Languages\\File Extensions\\.cpp' instead.