Herobrine is not a Ghost.
That's the stupidest thing I have ever heard however I do believe in Herobrine.
I have saw him in the nether and decided to start developing a mod so I could
see into the minecraft scripts and see if I could find anything about him.
Most of you probably havn't seen the minecraft scripts, but I can tell you
there are so many scripts, it will take weeks to have searched through
ever script for Herobrine.
The point is, I believe he IS in the game, but was added by notch to make it more fun.
I do not believe he is a ghost, he is but a script.
Plus I have a theiry about how notch may have added it without showing it in the scripts.
Skins is my answer.
The player model is already in the code so all the code is there.
The only code needed is the code making him spawn, which could
just be a script they named "Mob" or something like that so people
wouldn't think to look into it.
When Notch was putting "Removed Herobrine" on the update logs I think he is just milking it and making Herobrine more popular.