UPDATE: We now found the original source over at German Automobilwoche saying that Porsche will bring the diesel Porsche Cayenne mid of 2009, based on a V6 TDI from Audi and delivering over 300 bhp! The decision has been made against V8, V10 or even the V12 TDI due to weight concerns, therefore they will modify the current 240 bhp V6 TDI to deliver more than 300 bhp.
According to this Dutch source, citing German Automobilwoche, Porsche is working on a diesel version of the Porsche Cayenne. That would be a small, but nice revolution for Porsche.
Source: AutoTelegraaf, picture by Porsche