Canon has patented some lenses with resin elements recently, they are plastic elements that go inside the lens where they will not be susceptable to scratching...
So far, as far as I know, they have only been used in the AF unit of DSLR's, not in a EF lens.
In their Technical Room, Canon states
Canon uses four different type of aspherical lens elements now depending on the purpose;
1. a ground and polished glass aspherical lens element.
2. a molded glass aspherical lens element.3. a molded plastic aspherical lens element produced by a high-precision molding technology.
4. a replica aspherical lens element, ultraviolet-light-hardening resin layer on a spherical glass lens element.
There are plastic elements in some of the EF-S lenses (which are a subset of EF lenses) - the EF-S 18-55mm f/3.5-5.6 USM Technical Report
states that a molded plastic aspherical element is used, and I'd bet one is used in the IS version of that lens, as well.
Interstingly, that same technical report corrects a misconception which I suffered from (and Wikipedia
suffers from the same misconception) - that the "S" in EF-S stands for "short back focus." According to Canon, the "S" in "EF-S" comes from "Small image circle."