This is chainguard on the 2009 Lime series bikes, from the big T. This particular chainguard is also lime-colored, but the brightly colored portion is actually a removable applique (as are the lime-colored parts of the hub-covers and some other bits). Trek calls these large rubber bands Peelz(TM), and you can buy sets in a variety of colors. Presumably matching Trek's 2009 "casual cycling" clothing line and helmets.
Here's a shot of a whole Peelz Color Kit, in my favorite color. The two hard-to-identify items are a "top tube pad [and] seatpost collar QR grip."I know it sounds like I'm mocking this feature, but actually I like it. If it gets someone on a bike, then it's got something going for it. Now, there are real issues brushing up against Shimano's Coasting concept. They'll get a post of their own, I think.