The bitcoins in Genesis block are not spendable (but this did not prevent some well meaning halfbrains from sending additional coins to that address). But the private key for that genesis block exists (or at least existed) so Satoshi could prove his identity at any chosen moment if he wishes and if the key is not lost. I wonder if this is a bug or Satoshi was so bright to include this intentionally?

edit: Damn, my post was so similar to posts above even without reading them! Have we become drones in a hive mind?