I don't know who is the greatest (I do know MJ was the most transcending) but I do know that LeBron (Please god stop spelling it LaBron) is one of the greatest already.
Lebron is just in his prime now so it's very difficult to accurately place him. He has shown that HE can be the man and bring a team to the championship (did it by himself in Cleveland). He has shown with enough talent he can lead a team to win it all (Miami). He CAN take over a game (you're just bullshitting yourself if you don't agree).
His body, speed and size could be dominant in ANY era. Would have loved to have seen him in the late 80's/early 90s. Then again, I want to see him get multiple rings.
I agree that right now he's not the greatest, yet. Oscar, Wilt, MJ and Kareem all have a say in that. But if he finishes his career the way it has started then I easily think his name will be in the discussion if not in the lead.