Not only that.
Carbon fiber facts:
- Requires thicker construction for the same strength, compared with Duralumin, which has a strength comparable to some steels
- Not very shock resistant. Breaks easily.
- Does not conduct heat as good as aluminum or copper. Aluminum is the second best heat conducting (widely used) metal after copper.
- As mentioned above, recycling is difficult and not cost effective.
- Some forms of sturdy designs are impossible with carbon fibers.
- Carbon fibers are probably more expensive than "standard" duralumin.
- ...
The use of carbon fiber is quite popular for certain things. Tripods and bicycle frames use carbon fiber quite often because it's light. I don't know that it would significantly impact the weight with a laptop. The macbook pros and airs don't use a huge amount of aluminum in their end designs. They appear to be milled down from a larger block which can be expensive, but the lack of curvature in the design probably helps considerably there.
Like you were saying, a carbon fiber shell would mean you could rest it on your lap without feeling much of the heat. It would still need to be directed away from the components, so it's possible that the machine would require greater density to allow for larger fans, but I'm just speculating here. The real cost in either of these things is processing, and the cnc design does cost more to produce than the older plating design, but Apple raised the price at the time to compensate for this and retain a comparable margin.