[sperry]

Well, here's the rule:

The firewall, roof, doors, rear quarter panels, floor pan, trunk lid (if any) or rear hatch (if any) must remain stock. Holes may be drilled to allow the attachment of body kits, etc. This specifically allows replacement hoods, front fenders, wings, front and rear facias, side skirts, and fender flares as per Section 14.

I think the rule was designed to allow mild areo/cooling improvements and stuff for "looks", like body kits, but to avoid people converting the entire body to a fiberglass shell. A c/f trunk lid should be legal according to the "spirit" of the rule, IMO. It doesn't offer any huge weight savings, and as Phil mentioned the weight reduction is on the wrong end of the car. The c/f trunk is more of an asthetics mod, and I wouldn't protest if I got beat by someone w/ a c/f trunk lid.