Slush's Pool, the second biggest mining pool around, is currently down, allegedly due to a DDoS attack:
Pool is under DDoS attack. I'm working on recovering the service back to operation.
This means the entirety of its miners are currently offline, and this for sure lowers the difficulty of gaining 51% of the total network hashing power.
Now, I don't know anything about the hardware/software design of mining pools, how many servers do they employ to centrally manage all miners, how much bandwidth do they have, etc.; but I'm quite sure that nuking one of them would require a lot less computing power than actually obtaining more than 50% of the total Bitcoin network hashing power under its normal operation.
Thus, the question: aren't big mining pools a critical point of failure for the Bitcoin network? If a malicious attacker was to place several of them offline, all of their miners would be instantly disconnected, and then a not-even-so-big mining farm (or maybe a rival pool which engineered the attack in the first place...) could easily overcome the whole network.
How much likely is such a scenario?
How can it be avoided?
Should the Bitcoin community invest more into protecting pools, or, better, make a joint effort into moving from a "few big pools" model to a "many little pools" one, in order to be safer from similar attacks?