Je pense que...
I used to be head of IT at a city firm, was director in charge of the British government's secure network to monitor investment banks, and have done time as a tactical developer in several banks, but not SocGen. So the rest of this piece is inference spiced with direct personal communications as a headhunter with people who discreetly shared unattributable insights over fine wine or shouted them across mediocre lager. Short version: This is informed guesswork.
There have been the inevitable articles on "warning signs" that should have been spotted, and it's hard to argue with that. But to put that into context, everyone who has worked in trading firms has seen position reports that imply that you've accidentally bought up Microsoft, swapping it for your stake in SCO and an option to buy a goat.
Kerviel could explain these signals away because he had worked in the mid office responsible for risk management. When asked why he apparently had a huge position, he would easily have been able to say "sorry, when we built that report, we got it wrong in the way we handle failed trades", and maybe even offer to fix it. For a given value of "fix", of course. Certainly he would have enough excuses to get over the head of a manager chosen on the golf course (you know who you are).
How many passwords would you like M. Kerviel?
SG's official statement claims he stole passwords, but this is highly unlikely to be entirely true. As a mid-office developer he'd be freely given a pile of passwords and access rights. I would actually be quite surprised if all of these were taken away when he was promoted; and as for the passwords of his colleagues, they are shared far more often than the security policy of any bank would permit. I have seen junior staff on trading floors told to log on as other people (no, I'm not saying where, but it's common). The way it works at most firms is that as his new job required more access, this would have been added to his profile, but no one would have asked whether he should be taken off the mid-office servers.
Many organisations deal with "permission creep" by randomly deleting access every year or so, and waiting for people to complain (yes really, it's not just your firm, happened to me more than once). But a common bad technique is to embed usernames and passwords into applications, especially Excel report sheets. These tend to be powerful administrator or developer accounts, granting unlimited access with little or no auditing. I have seen what happens when these accounts are deleted without asking the developer, and it is not pretty. Who, me? Would I do such a thing?
Of course this rarely happens to "important" people, and the food chain at most investment banks has even junior traders like Kerviel in the top tier, so I'd bet money no one even asked why he still had the old access rights. Realistically, being able to do extra useful things and sort out problems with "lower" areas would have made him a more valued member of the trading team, so anyone trying to take his access away would therefore have had a fight on their hands. Such is the menial position of IT in most firms, most would not even try.