Hi,
I had the same problem. What you need to do:
1. Go to "accounts and sync"
2. Click your whatsapp account and remove your whatsapp account (do not go to whatsapp application and delete your account from there!)
3. You should not be able to remove your whatsapp account from there, but that's fine because that's not the point.
4. Go to your whatsapp application and refresh your contact list.
5. Go back to your phone contact, go to contact display and check whatsapp contact.
6. Your hidden whatsapp contacts should be back there in the phone contact list.
Cheers...