Last night's episode really leaves me wondering something. If Michael is using the computer to talk to Walt and the guy on the reel STRICTLY forbade any communication on the computer and the computer ONLY responds when it's time for the code, could "Walt's" responses be false?
Also, the Guardian was shown again. It was the black lightning clouds that proved it was only as scary as the person it was hunting made it and with Eko's past, he feared nothing.
Following the episode, we officially know (for the most part) why every major character is there.
Jack - Felt to blame for the death of his father; brought to the island to realize he is a capable human being able to make decisions; The Doctor.
Kate - A convict who killed her (step?) father; brought to the island to experience freedom from the stigma of prison and to give her a new start; The Convict.
John Locke - A parapalegic (never revealed why really) who immerced himself into a world of wargames to escape reality; brought to the island to realize he could always walk once he freed his mind; The Prophet.
Michael and Walt (Though separate characters, I will count them as one) - A father who never met was allowed to be with his son until his wife died, due to the estrangement, Walt never developed an attachment; brought to the island to rekindle and strengthen their father-son bond; The Father and Son (Walt could also be called The Mystic due to his possible psychic powers and Michael would be The Scientist of sorts with his ideas for the boat and knowledge of the GPS on the boat).
Charlie - A burnt-out, drug addicted rockstar who had a bad falling out with his brother; brought to the island to clean himself up of heroine and take the responsibly (over Aaron); The Rebel
Mr. Eko - A mercenary who was taken from his brother at a young age, felt to blame for the death of his brother and feels the burden of his death; brought to the island for closure on his brother's death and to face the demons of his murderous past, to find God; The Priest.
Anna Louisa - An ex-cop who killed the man who shot her and killed her unborn child; brought to the island to show her that all mistakes can be forgiven with time (Shannon's death); The Scapegoat.
Said (My favorite character) - Years of work as an Iraqi soldier, Said is harden and heartless; He was brought to the island to soften up and realize that the people he killed and hurt, were human beings; The Soldier.
The characters all have a definate reason as to why they are brought to the island. I think in the next few episodes (particularly from the trailer of next week's episode) the others will be revealed as to how they got there and why they went so crazy. I really wish that at least for the season finale they do it commerical free.