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Yes, if by earlier you mean a few minutes earlier (it would have to have a 50+ BTC fee to make it worthwhile for miners to double-spend a transaction that already made it into the last block).
Two zero-confirmation transactions essentially happen at the same time as far as the network is concerned; if there was a good way for everybody to agree that they happened in a particular order we wouldn't need the blockchain. I assume that miners will take the transactions that give them the greatest short-term profit; that may not be correct (there might be a lot of miners interested in the long-term health of Bitcoin), but I think it is the safest assumption. And I think if we design payment protocols around that assumption they will be more secure.