Estimates put his or her holdings at 1 - 1.5 million bitcoins. That could be slowly leached back into the market. The person could be dead. The wallets and/or keys may be gone or... They could cash out and collapse the market. Not that it would be a permanent killer but I can't help but worry about it every time the price soars.
Coindesk: How Dangerous is Satoshi Nakamoto
Business Insider: Satoshi Nakamoto owns a fortune
Sergio Lerner Analysis
And what about the 144,000 bitcoins seized by the FBI when they shutdown silk road?