There is also another point that has not been discussed yet: Emerging markets.

Going forward, Porsche's sales growth will come from so called emerging markets (20% of unit sales today). This side of the business has been growing very rapidly since the Cayenne introduction (sales excl North America & Europe 14% in FY 03/04, 18% in FY 05/06)

In those markets, most of the time, the "sports car" segment is non existent for many reasons (safety, road conditions, social issues, ...) therefore for Porsche the only way to address these markets is to have products like the Cayenne or the Panamera.

The Cayenne represent 35% of Porsche total sales, in some regions it's much higher.

Middle East: Cayenne 68%
China (excl. HKG) Cayenne 81%
Russia: Cayenne 84%
Latin America: Cayenne 64%

Dealers (in emerging markets) to be proftable must have more than one product to sell hence the crucial need for the Panamera.

Also in some countries like China, if your are wealthy, you have a chauffeur. I don't picture anyone sitting in the back of a 997 TT with the chauffeur driving the car ....

From what I understand the Panamera price point is likely to be at the end of the 911, hence much higher than the Cayenne.

Bottom line the Cayenne will become the Boxster/Cayman of the 4-door line-up while the Panamera will be the equivalent of the 911