Consider for example, Second Life. You buy stuff in Second Life (clothing, objects, property, etcetera). Now consider if we decentralized that environment… made it where anyone could run the server software (this is already possible with OpenSim). Now we need a viable form of commerce that is not biased to any particular party. This is where BitCoin shines
I'm guessing he is going to make an MMO, allow others to run servers, and sell items via bitcoin. Of course, this doesn't explain why he would need a stock of bitcoins... this implies he is going to be buying something.