Porsche will apparently exhibit their current Porsche Cayenne Hybrid prototype at the IAA 2007 in Frankfurt next month.

Porsche announced that they were working on a Hybrid Cayenne back at the IFA 2005. Porsche uses a parallel concept for their hybrid drive. Currently they use a V6 engine that works seamlessly with an electric engine powered by a block of batteries.

The fuel efficiency of the Porsche Cayenne with Hybrid drive is currently 24 miles per gallon (9,8l/100km). The final version of the Porsche Cayenne Hybrid is supposed to be more efficient.
The advantage of the parallel hybrid drive is the possibility of running on the electric engine outside of city use. You can glide with speeds up to 120km/h (75mph) on the electric power - ideal for U.S. highways but not for the German Autobahn.

The target release for the first hybrid Porsche is still 2010.
See also the Porsche site (German).