Originally Posted by lvan
I was not quite sure what he meant but, I suppose the IS makes it focus better in limited light.
IS allows you to use a slower shutter speed(1-3 stops) in low light, to take better/less blurry shots handheld. It's still always better to use a tri-pod, in which case IS does nothing. It doesn't really have anything to do with the camera 'focusing', but since without it, images are easier to blur due to the camera movement and longer shutter, I can see would you would think that.
If you plan on shotting alot in low light, i'd go for the 2.8 non-IS vs. 4f IS, assuming the prices aren't far off.