Recently I have built a FAQ for the Canon EF 100 400 L IS and in one of the answers I brought up the subject that the push/pull zoom of the Canon EF 100 400 L IS sucks the dust right into the sensor.
I got some emails from owners confirming that they have had no dust problem with that lens whatsoever which, by the way, was a big relief for me since I am expecting mine to arrive any day now.
Recently a fellow photographer did some scientific tests with his Canon EF 100 400 L and here are the results
Olli sealed with double rubber a plastic bag on his lens to check the amount of air going in and out at every pump
notice the volume difference of the bag.
The original thread can be found at Photography-on-the-net forum