Well, it seems that the stabilizer on the 17-85 is working, but that it's much harder to "see", and you can't "feel" it as easy as on the 70-300 where the difference is very visible. I guess that as the length of the lens changes, the motor needs to work harder and you can feel it more easily. Thanks,
The reason it's not as visible on the 17-85 is that the "impact per time" is much less. Your visual perception doesn't change its frame-rate/shutter-speed when you change the focal length of the lenses and as such, wider angles are much less shaky in your eyes. I bet that if you compare the 17-85 at the long end and the 70-300 at the short end, the visual efficiency of the IS (in the viewfinder) will be similar.