While I doubt that Canon has a sweeping 'no lemon' policy beyond what is offered in their basic warranty language, I will say this: photographers who are Canon Professional Services members are offered services well above and beyond those that are offered within the basic warranty language.

As someone who over the years has had equipment lens malfunctions both as a non-CPS member, and also as a member I've personally experienced the difference in service level...

I had a regular warranty claim several years ago which essentially went the 'standard' route with Canon of 'ship the lens back to us at your expense and we'll repair or replace it. You'll be without your lens for 2 or 3 weeks while we take care of it.'

As a CPS member, on the other hand... Last year I had a lens go out on me in the middle of a project with a tight deadline and their response was one of 'no-questions-asked' next-day-air shipping of a brand-new duplicate to the malfunctioning lens at their expense. Upon shipping my defective lens back to them (again, at their expense), I was told that my lens's electronics did indeed have a problem (which was causing AF 'scrubbing' problems) and to simply keep the lens they had sent me in exchange 'with their apologies.'

I get the feeling that the customer service reps with the CPS program are given pretty broad leeway in how they can handle things for the members... Maybe that's where the notion/idea/rumor of a 'no lemon' policy has stemmed from?