Its an interesting concept, I've thought about it once or twice since I started shooting with my 100mm macro lens. Unfortunately, I doubt it among Canon's top priorities for future lenses. I'm not sure IS would remedy my focusing problems, I tend to have to move closer or further as f/2.8 is kinda narrow to focus in.
IMO IS wouldn't help macro photography very much. The issue isn't camera shake, it's the constantly moving plane of focus. IS doesn't stabilize your forward/back movement, nor does it stabilize the movement of the subject. If anything, a really good AI servo, fast enough for macro work, would be a step in the right direction.
I would love to have a macro lens with IS. Although recommended, I often do not use a tripod. I would love to have IS especially on the 180L to enable handheld 1/60s to 1/100s shutter speeds enabling more ambient light to light from flash ratio.
I have been waiting for one for yrs now. Now that Nikon has one, I have a feeling that Canon too would come out with one in an year or so.
Pop photo's review of the Nikon 105 macro (called the micro?) says that the image stabilization is most effective starting at ten feet, which is a little far for macro They did note that they could still get 1-2 stops of improvement at 1:1.3
It's about $200 more than the non-image-stabilized version too.
I think IS might help since even with a good tripod and remote shutter release. There are still some sort of vibration especially when u are using longer focal length or extension tubes or there aren't enough light to use a higher shutter speed. However, I am guessing the current IS of Canon probably not designed initially to be used for Macro. I am sure some forms of IS can be done on a macro lens to help. Unfortunately, Canon is heading toward the bigger consumer market. Most people might not do macrophotography and might not spend a few hundreds more to get IS on that macro lens if they even buy one.
I personally wouldn't pay significantly more for a macro lens with IS. Most of my macro work is already done on tripod anwyays. And, as stated above, a bigger problem is forward/backward movement, because that can shift the plane of focus. As far as I know, IS only counteracts lateral/vertical movements. The IS would probably only be useful if using the lens as a telephoto prime.