I don't know how useful IS would be for macro, but I hope Canon will produce it so that we can find out for ourselves. A 180mm macro IS would be sweet - if it doesn't weigh a ton and cost a couple of limbs or some other body part.
I got a friend using a 100mm Macro on a 400D..... which makes it 160mm after the crop of x1.6.
He complaint all the time that its quite hard to stable the lens due to how close the subject is..... But like you said, I too hope that Canon would make a MkII for the 'old' EF 100mm f2.8 Macro. :bsmilie: