After months of speculation, Porsche has finally acknowledged that the Cayenne will be available with a diesel engine, starting in Feb. 2009. They didn't develop it on their own, it's the well-known Audi/VW 3.0 V6 TDI, which will be used seemingly unchanged.
Link (sorry, German only):
Some months ago it was Subaru, now it's Porsche's turn. The number of car manufacturers who don't offer a diesel option is ever-shrinking...
Please don't flame me for calling the Cayenne a car.
And for anyone who is interested, these were the very first Porsches with a diesel (no kidding):