You can buy a new key for Mac OS X. Its specifically stated on http://www.apple.com/quicktime/pro/faq.html about halfway down the page that a Windows key won't work on Mac OS and vice versa.
I bought QT Pro 7 for my Mac, and was disappointed it didn't work for Windows.
But, TBH... I hardly use the Pro extras on Windows, so it's not too bad for me.