Oh, also, most L lenses "require" a filter to be screwed in to "complete the weather sealing", so something interesting to note while you're outdoors.
That's actually debatable for 'most' L lenses. A filter is definitely required to complete the sealing of a few lenses - 3 current lenses, to be exact. Lenses with exposed internally moving focus or zoom elements have that requirement listed in the manuals - 16-35L II, 17-40L, and 50L. A filter is definitely not required for the super telephoto lenses, since they do not take front filters, but they are weather sealed lenses. For all of the other L series lenses, there's no 'official' statement that a filter is required to complete the sealing. However, in an email exchange with Chuck Westfall he recommended using a filter.