The computational power to attack the network grows significantly faster than the price of Bitcoin (and this is pretty much guaranteed to continue, by design). Attacking Satoshi's coins would be among the hardest, because you would have to attack the very earliest parts of the Blockchain which have tens of thousands of blocks protecting them. Half a billion dollars wouldn't come close at the current difficulty level.
Further, performing this attack would invalidate all Bitcoins created after that point (which is the vast majority of them), so the Bitcoins you stole would be worthless.
To add to this: The "official blockchain" is the one with the most computational work expended to create it.
A 50+% attack operates by rewriting history back to a certain depth in the blockchain by providing a new chain of blocks with more computational work, back to a certain point.
I think the bitcoin devs also put in certain "checkpoints" just to be even more certain... i.e. blocks that are "officially permanent" (which also makes all prior blocks in that chain "offiically permanent").
Ah yes, I forgot about checkpointing. The reference client has certain blocks hardcoded in it to signal the official chain. Only transactions in blocks made after the latest checkpoint can be overridden in a way that the reference client would accept. So to steal Satoshi's coins, you would also have to get people to switch to your own client with different checkpoints. At this point, it really wouldn't even be Bitcoin anymore.
Further, performing this attack would invalidate all Bitcoins created after that point (which is the vast majority of them), so the Bitcoins you stole would be worthless.