i've been mining for a month with deepbit-slush and btcguild, and id say IMHO that noticeable DDOs happen like twice a month for the bigger pools. I do however keep my mining profiles with 3 or so pools, and remotely log (using free logme in via any internet connection) into the miner and switch if X pool is down. Would be nice that the mining gui would fallback to other pools if X is down.