It depends on who Satoshi Nakamoto is.
If it is a Ahole like Craig W. then the price will plummet, because investors will fear a million coin dump from a person that are clearly out to sabotage Bitcoin. <BTC>
Also, if it is some 3Letter agency, people will sell their coins in fear of the possibility that Bitcoin was developed for some surveillance experiment.
The opposite will happen if it is someone that are deceased and that has no access to the Private keys of those millions of coins or if it is a reputable person in the Bitcoin community. The best option is for Satoshi to remain anonymous, because no matter what happens, the Bitcoin price will be influenced.