Porsche's problem is they ( and this refers to the 911) produce far too many and charge far too much for them.
Price:
I think per car produced they have the largest profit margin of any manufacturer. They need to rethink thier pricing. When you spec up a Carerra S to decent level it comes in around 80k- that's too much for a 'standard' 911.
Volume:
There are just too many produced. There used to be a time a 911 was a good investment, decent running costs and a good residuals at the end. The residuals now just doesn't apply. With the R8 and V8 in the marketplace the running costs for alternatives isn't that different now, but they'll both be worth more than a Carerra S come 3 years down the line.
Competition:
Over the last couple of years the competition has grown considerable with the likes of AM V8, Audi R8, M series, AMG's, Jag XK, Maserati's, GTRs. I am sure you'll see that the 911 market share has shrunk in the last decade.
Don't get me wrong, I love 911s- heck I own one!, but while being the best drivers car ( in my humble opinion), Also think it would be the most economical as well as the cars above are all gas guzzlers. Personally, I wouldn't change the look of a 911 at all (but the interior needs a serious rethink). however, I do think the overall package however has diminished.
No doubt the current economic situation is having an effect along with eco concerns, but I think if you are looking for a 2+2 everyday supercar you'll struggle to beat a 911- but I ask how many 911 owners use thier car this way?.
I'm sure most 911 owners will use it as a 2nd/3rd car.