Everyone who plays poker online shares these same concerns, so it's not surprising that the largest sites care deeply about their reputations (especially since they make so much money playing it straight up).
That said, there has been at least one major incident where one of the original employees of a poker site used a "superuser" account to see his opponents' cards at the high stakes tables and win lots of money. The players figured it out based on his statistically improbable win rate and an accidental release of the full hand history of one of his tournaments.
"This Grey Cat person was new. And at first, he seemed like a live one. He seemed terrible," Witteles remembers. "He seemed to play crazy. It seemed like he was giving his money away. Except the only thing was, he wasn't losing. He was playing in a style that was sure to lose, but he was killing the game day after day."
It seems that discovery of this cheat was mostly due to the newbness of the cheater.
If skilled player possessed backdoor access to poker system and used it wisely, peeking in only from time to time when it's necessary and might go unnoticed he'd be rather undetectable.
