While we now know more of Christian, there are some odd discrepancies as to his various appearances, to Jack and to other characters. The most intriguing one being his appearance to Jack at the hospital in LA.
Here are Christian's appearances to other characters - in chronological order!
WHO : EPISODE : WEAR : SOUND
Vincent : So It Begins : Suit and white tennis shoes : no sound
Called Vincent and told him "Listen. I need you to go find my son [...] I need you to go wake him up. He has work to do"
Jack : Walkabout/White Rabbit : Suit and w.t.s. : no sound
Jack saw different occurrences of Christian on the beach and followed him in the jungle. Saw a silhouette and heard ice tumbling and followed him to the caves.
Jack : A Tale of Two Cities : (voice only) : (voice only)
Heard Christian say "Let it go Jack." over the the shark tank intercom.
Hurley : The Beginning of the End : Suit and w.t.s. : no sound
Saw him in the cabin, sitting in Jacob's chair. Ran from the cabin then it appeared in front of him. He closed his eyes saying "There's nothing here" and after opening his eyes, the cabin was gone.
Claire : Something Nice Back Home : Dark blue shirt : no sound
Claire woke up to find Christian cradling Aaron and called him Dad. Miles admits to Sawyer of hearing them.
Locke : Cabin Fever : Dark blue shirt with boots : no sound
After Hurley finds the cabin, Locke goes in and asks if he's Jacob. Christian answers "No, but I can speak on his behalf." Eventually Locke says "I'm here, because I was chosen to be." To which Christian replies "That's absolutely right.".
He then notices Claire who tells him "Don't worry, I'm fine. I'm with him." and after asking about Aaron, Christian tells him "The baby's where he's supposed to be, and that's not here. It's probably best that you don't tell anyone that you saw her."
Michael : There's no Place Like Home (2) : Dark blue shirt : whispers
When the freighter is about to explode, Christian appears to Micheal, tells him
"You can go now Michael."
"Who are you?"
Locke : This Place Is Death : Dark blue shirt with boots : no sound
Appear at the bottom of the well and helps Locke find the Dharmakackra and turn it.
Jack : Something Nice Back Home : Suit and w.t.s. : no sound
In the hospital (flash forward) when Jack was working late. He was on the couch and called "Jack." He leaned forward as Jack was walking towards him but the Jack got distracted.
-Given the information we were given in The Last Recruit, this is the most intriguing appearance of Christian. But are we to believe the White Rabbit occurrence of Christian was truly the smoke monster?
Sun & Frank : Namaste : Dark blue shirt with boots : tika tika AND whispers
When Sun and Frank land on the beach, they hear the 'tika tika' and see the trees moving. When they get to the processing centre, they can hear the whispers before Christian appears and shows them the Dharma recruits picture.
It seems like we have stumbled upon two different versions of Christian:
 Wearing a suit and white tennis shoes; and,
 Wearing simply a dark shirt and boots.
What to make of this?