Was glancing through a couple month old issue of a Porsche magazine I get, and was re-introduced to this neat concept from Oakley Design (UK). Working with Airbus, they now offer a true carbon fiber cage for Porsche owners. It meets full TUV standards, and is currently going through the approval process for FIA. Apparantly they use some sort of anti-splinter process to ensure the cage does not shatter into shards if it is subject to an impact. Weighs just over 24 lbs complete. The same cage made of steel is 120 lbs according to Oakley. Neat stuff indeed!
Oakley Design Carbon Cage (as installed in their GT2) - pic courtesy of Evo Magazine
Kwame
Z1 Performance
Jonathan
Corner Balance 
I was thinking about this just the other week wondering if its ever been done. It will be interesting to see if it pass’ FIA standards