The SD400 has a fast enough shutter speed,
you have no way to control it accurately.
This is typical of any point-and-shoot camera.
That is one of the reasons that manual controls are important. When shooting action/sports, you want to use the manual controls (shutter priority) and set the shutter speed to something like 1/500th second for starters. The camera will select the correct aperture setting for the correct exposure.
The faster settings require more available light.
When you are shooting in dim lighting conditions, you may have to select a slower shutter speed. It depends upon the available light.
So if you plan to use the S2 IS in daylight conditions, it can capture the action without blurring the subject.
Of course, holding the camera steady is your job.
The S1 IS shutter lag was not so good (0.8 second).
We will have to see some reviews of the S2 IS to find out if Canon improved that situation.
With the correct lens selection, any of the DSLR cameras will do better on shutter lag and the ability to take action photos in dim lighting conditions. But it will cost you much more.
I have a pocket camera the SD400 and want to get a camera with a higher shutter speed and lower shutter lag for sports. Should I just get the S2 IS, or go with the 20d or Canon Digital Rebel XT. Will the S2 IS be able to take sport pictures without them being blurry?