I've read a fair amount of Satoshi's posts and I remember that he didn't envision mining pools.
ribuck's description is spot on.
Pool operators can modify their getwork to take one additional parameter, the address to send your share to.
The easy way for the pool operator would be to wait until the next block is found and divy it up proportionally as:
user's near-hits/total near-hits from everyone
That would be easier and safer to start up. It also has the advantage that multiple hits from the same user can be combined into one transaction. A lot of your hits will usually be from the same people.
The instant gratification way would be to pay a fixed amount for each near-hit immediately, and the operator takes the risk from randomness of having more or less near-hits before a block is found.
Either way, the user who submits the hit that solves the block should get an extra amount off the top, like 10 BTC.
New users wouldn't really even need the Bitcoin software. They could download a miner, create an account on mtgox or mybitcoin, enter their deposit address into the miner and point it at anyone's pool server. When the miner says it found something, a while later a few coins show up in their account.
Miner writers better make sure they never false-positive near-hits. Users will depend on that to check if the pool operator is cheating them. If the miner wrongly says it found something, users will look in their account, not find anything, and get mad at the pool operator.
Right, that was just weeks before he disappeared. Prior to that mining pools were not something that was on his radar. He clearly didn't take into account the dangers of mining pools controlling large percentages of the network.
At that point mining pools were a novel addition to bitcoin and the full impact of their existence obviously wasn't considered.
He was aware of the concept of pools, and he was aware of the possibility of someone gaining more than 50% of the hashing power:
The incentive may help encourage nodes to stay honest. If a greedy attacker is able to
assemble more CPU power than all the honest nodes, he would have to choose between using it
to defraud people by stealing back his payments, or using it to generate new coins. He ought to
find it more profitable to play by the rules, such rules that favour him with more new coins than
everyone else combined, than to undermine the system and the validity of his own wealth.
I suspect that the concept of a pool acquiring more than 50% of the hashing power didn't go over his head. It's clear that he expected the financial incentives to protect the network regardless. At least with a pool, the people leasing their hash power to the pool can choose to leave. That makes a pool with more than 50% far more secure than Satoshi assumed a lone individual with more than 50% would be.