DDoS on big mining pools: a quick way to 51%?
There's no cooperation involved in mining, and they won't be trying the same hashes because (for example) they have different transaction pools and (possibly) different destination addresses. Mining pooling is just like non-pooled mining, except that the revenue is spread over time instead of getting blocks of 25BTC.

Even when the pooled miners have a backup set that sends them to the largest pool (BTC Guild), that's acceptable for short periods of time. But miners are still people, and if something like that happens they'll quickly correct the issue.