Dust on the lens was never an issue... But dust on the sensor is and always will be
-> On film one has a new frame everytime the shutter is released, but on the digital the same stationary sensor is 're-used' for every shot...
I have the 100-400L too, but the potential is always there over lenses (with internal zoming) that do not extend or retract.
(All anyone has to do is to take the rear lens cap off and feel with your hand all the air behind this lens when its being pumped)