Awesome idea...
I was thinking about this recently that Apple should come out with a carbon fiber version of their laptops. Long battery life and lighter weight go hand in hand. No need to go crazy with faster GHz processors and just improve the battery life, weight and heat management and you got a great machine. Knowing Apple, they'll slowly introduce carbon fiber by replacing small parts in their laptops and eventually transitioning to carbon reinforced unibodies, then, maybe, full body machines.
And also, cost will have to be managed throughout this transition. With any new improvement there will be an associated up cost and as manufacturing cost decline, these newer laptops should be more affordable. Hopefully, Apple will keep the price of new laptops comparatively the same as they are now (or slightly more) for the top of line models.
Lighter weight MBPs will definitely be coming to an Apple store near you. When? Who knows, but it's coming.