Thanks Sean, I've heard of Canon's variablilty in quality control before, but I've obviously been lucky with the lenses I've bought. We have a 10-22, 24-70, 24-105, 70-200 and 100-400 for the 20D and 5D, and all of them look very good when comapring them with friends'.
I think a little in-store comparison between 16-35 and 17-40 (and each individual example of these that they have in stock) might be just the ticket.