If you look at deepbit, when it was hit by DDOS, everyone switched. There was no "loss" of pooling.
There was indeed a noticeable effect when deepbit first went down to the DDOS.
In the hour after the attack the network only solved 2 blocks. It improved from that point on, but it still wasn't great. That certainly isn't desirable or what I would call healthy.
127555 2201f7d916... 2011-05-29 22:41:30 43 1472.65095723 21.143
127554 15fa32c760... 2011-05-29 22:13:38 37 877.09812426 23.145
127553 1de06a4a0a... 2011-05-29 21:46:22 47 2052.27689526 17.552
Miners are slow to notice, slow to react, and I suspect some didn't bother switching at all.
I wonder how many miners would even bother (or are capable) to set up back-up solo mining if a coordinated attack was able to shut down 4-5 major pools.