You were right about the front elements and I was wrong. Thanks for letting me know. Below is an e-mail from Chuck Westfall of Canon. (Emphasis mine as it may be relevant here.)
Hi, Artie:
The front elements of current EF IS super-telephoto lenses* are meniscus lenses made from optical glass with Canon's SSC multicoating. (SSC stands for Super Spectra Coating.) They are mainly intended to be protective elements, and as such they are fully gasketed. But the meniscus shape is also important in eliminating ghost images when used with EOS Digital SLRs. The front elements of the older pre-stabilizer FD and EF super-telephoto lenses were also gasketed and multicoated, but they were flat rather than meniscus-shaped, and prone to ghost images when used with digital SLRs. In all of these lenses, the front elements provide some protection against UV wavelengths, but nothing special.
*including: EF200/2L IS EF300/2.8L IS EF400/2.8L IS EF400/4 DO IS EF500/4L IS EF600/4L IS EF800/5.6L IS
Other white lenses like the 100-400L, 70-200L's, 300/4L's, and 400/5.6L accept screw-in filters (as you know), and their front elements are also multicoated but typically not gasketed. Canon does not officially recommend using UV filters on any lens, but they can certainly come in handy on telephoto lenses for impact, dust/dirt and moisture protection as well as varying amounts of haze reduction. I usually tell customers that if they're going to use a UV filter, make it a good one from a well-known manufacturer rather than a cheap no-name brand, keep it clean and be sure to use a lens hood whenever it's practical.
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