Hollow boards have been around in sailboards and surfboards for a bit. I built one a few years ago using 9 oz CF and 1/4" end grain e.balsa that was wayyyyyy stiff and heavy - close to 8 pounds.
There are currently two basic hollow carbon construction methods. Hydroepic and Kolstof used aluminum honeycomb on the deck and some form of internal stringer glued in. The boards are built in half's and the internal structure glued in and finally the two pieces glued up.
The other is the aviso, which builds using high density foam skins inside a mold, that is layered up on a bladder. Once the mold is loaded the bladder is inflated pressing the components against the mold surface.
The Aviso website:
http://www.avisosurf.com/TechAdvantage.html The Hydroepic website showing the cutaway for the internal frame:
http://www.hydroepic.com/techaeroflex.php The Aviso cross section: