I bought the registration key and entered it as per instructions however the Pro items are still grey'd out and I can't use it. Did I do something wrong?
Hard to tell since your description of what you did is a bit skimpy. The usual problems are:
1) Purchasing the wrong key verasion. That is to say, Windows and Mac keys are not generated for interchangeable use.
2) Failure to enter both the "Registered To:" and "Regisdtration code:" exactly as specified in your purchaser confirmation message. This includes things like a prefix (Mr., Dr., Prof., etc.) or suffix (Sr., Jr., MD, DDS, etc.) if used for your account purchase name.
3) Not pressing the "Tab" key to register field entries before closing the the QT 7 "Registration" window.
Not sure if you were referring to the instructions provided in the Confirmation message or online instructions for latest Mac OS X Snow Leopard, Linon, and/or Moutain Lion. Just in case, here is a link you can check: http://support.apple.com/kb/HT3678
I had the same issue as Gedster and I also followed Jon's instructions.
My solution was found via a call to Apple Support, great help by the way. They had me go to System Preferences, Disk Utility and Repair Disk Permissions. This cleaned up the permissions on my disk. I then re-installed QuickTime Pro and the Pro features were available.
Hope this helps!