I have just read elsewhere that it was officially confirmed by Canon representative that both of these lens:
- Canon EF-S 17-55mm f/2.8 IS USM
- Canon EF-S 10-22mm f/3.5-4.5 USM
are all L-glass. The only reason that it is not marked as L is because of EF-S mount. Canon has a policy that any L-lens should be a EF mount. Both of these lens were designed specifically to take advantage of cropped sensor and bring wide-angle L-series quality to cropped bodies. It is interesting, that these two non-L lenses have better image quality than any other L-zoom lens in the same focal distances.
In fact, only wide-angle L-primes have better image quality than these two lens.
Wanted to share this,
Nikita Ivanov.
http://www.flickr.com/photos/agilepixels/