tl;dr - There are are 3 likely suspects...
KNC, ghash or BTCGuild
I doubt it's Discus Fish since many of the people who would leave Eligius for calmer waters would not pick a Chinese centric pool
If it is not the above three, then it is something personal and/or jealously as the driving factor
I too have wondered who it would be and if it is not a private person or activist group that Whizz or Luke have pissed off then I suspect big business to be at the heart of it.
My suspicion would fall on the companies providing protection from this type of attack as many have the infrastructure to generate such an attack, as it is the only way that they can re-generate in volume the attacks that they protect against. It can soon be turned into the old protection racket:
Shop window gets busted
Racket says we can protect you from that
You say no
Shop window gets busted
You say yes
Shop window no longer gets busted