It seems that most diamond moderators have this hat on Meta. Many, but not all, are wearing it.
- http://meta.stackoverflow.com/users/3/jarrod-dixon has it but isn't wearing it
- It's not just SE employees, because Jeff Atwood has it.
Some don't (I think community-elected moderators):
- http://meta.stackoverflow.com/users/50049/tiny-tim-post is a moderator, but apparently doesn't have the hat.
- http://meta.stackoverflow.com/users/137537/boltclocks-a-unicorn
Jon Skeet does not have this hat, either here or on SO main.