Technically if you ddosed all the pools and went on to hit individual users with a coordinated strike by multiple botnets, you could then very easily 51% the network.
"Very easily"?
Using BTC-PoolWatch there are an estimated 14,000 miners who are part of top 15 pools. That puts total # of miners at roughly 22,000.
So you are going to DDOS every pool and then find and DDOS 22,000 discrete solo miners (or a significant fraction of them)? While not impossible I would hardly call that "easy".
Large pools are the weakest point of Bitcoin network. Concept like p2pool have the potential to completely decentralize pools providing no single point of failure (for each pool). A network of decentralized pools would greatly harden the network against DDOS attack.