IS Operation ?

While shooting some ducks over the weekend with the 100-400 I was having trouble remembering to switch the IS mode between 1 and 2 for still shots verses panning shots, ended up with them backwards for a bunch of shots. Then I got to wondering if there is a point in shutter speed that IS does not have an effect on general camera shake ? In which case it wouldn't matter what mode the selector was in.

It isn't about shutter speed really. If in doubt use mode 2. Some users have it in mode 2 all the time, some in mode 1 and do not switch. ALso try panning with IS off and see what happens.

Diver-Down wrote in post #5136764
Then I got to wondering if there is a point in shutter speed that IS does not have an effect on general camera shake ?

With no IS, you need a shutter speed of 1/focal length generally.
So, if you are shooting at 200mm on a full frame body, then 1/200s or faster and IS shouldn't be needed with proper technique. If you have a crop factor camera, multiply your mm x crop factor. So on a 20D with a 1.6 factor, you should aim for a minimum of 1/320s at 200mm. This is a general rule and will vary depending on situation and technique.