The only serious concern I've found with Bitcoin so far is the probability that Satoshi has one or two million stashed away somewhere. That would represent up to 33% of current bitcoins and 10% of eventual bitcoins.
The problem with this theory is that "one or two million [bitcoins]" is already worth a big chunk of money, namely 20-40 million USD. If your goal was just to get rich easily from your new currency invention, isn't 20-40 million enough? I feel like if he was going to just take his money and run it would have happened by now.