If challenged, I am not going to. And I am probably not going to donate, either. Here is why.
I think that it is an excellent cause to fundraise for, and it has been a wild success. Whoever thought of it is a genius!
So why won't I do it?
The challenge: I don't want to get dumped on by ice cold water. I understand that's the point--to push yourself to do something unpleasant. I also choose* not to spend my time gathering the necessary equipment and a videographer, and then posting it.
The donation: As I said, it's a very worthy cause. But there are many other causes that are higher priority for me, and I don't like being pressured into giving my money to a different one.
So I dearly hope noone nominates me. I will just ignore it if someone does. PLEASE, noone here do it, or else

*I've stopped saying/thinking "I don't have time" and instead considering things as whether I choose to spend my time on them or not.