The patoshi pattern, that o_e_l_e_o referred to,  is a "proof" that Satoshi mined at least about 1M bitcoins with the same machine.

Probably he mined more with other machines, who knows. What we do know, is that he has not touched any of the million coins, and probably newer will.

My guess is that he/she destroyed the private keys to those addresses, because he wanted to distribute the bitcoins fairly among people, and he probably thought that if he takes million coins to himself, people would think it "unfair" and it could work against bitcoin in the long run.
Or prevent bitcoin becoming popular.