If you google for 5D dust you'll find that some people think the 5D is a dust magnet. I can't honestly say I've ever had a problem with the 100-400 and it gets used a lot, including dusty environments. Maybe I've just been lucky :shrug:
Just looked at my new 24-105L, there is a rubber dust ring to stop the problem, says in the manual it can be replaced when worn.
Mega Peeved or what, is there a known solution for the 100-400?
Jim