Carbon fibre would be cool, but prohibitively expensive. I used to have a Delta Kayak made out of special Thermoformed plastic. It was excellent. Stiff, and crazy durable, and VERY lightweight. I'd LOVE to see a mirage drive boat made out of this stuff! It wouldn't be much more expensive than rotomolded plastic either.
EDIT: Well... the material wouldn't be much more expensive. For Hobie to bring in the material, machinery, and expertise, it would certainly cost a lot more. But a partnership between Hobie and Delta Kayaks would be my dream boat.
(Are you listening Hobie?)
[youtube]http://www.youtube.com/watch?v=nkdaqwQxiFc[/youtube]