Is content protection enabled? The only other thing I could think of is adding a new contact with just the pin and after saving it, update the other information around it. Are you getting the un editable contacts from BES or Lotus or something?
Are the contacts you can't edit coming from BES or Lotus? If they are coming from some type of work account you may have to try editing them on your desktop client and they would sync that way. Other than that, I've never seen that happen... You might have to create a new contact with the info you are trying to edit. I see your contact list getting confusing, though. Wish I knew...