well, if this wouldn't show up after more than 2 years of development and couple of grand more in comparison with the IS, it would be very bad for audi :)
The 6 cylinder 3 litre diesels are the best. I've driven an Audi A6 3.0 TDI, BMW 530d, 535d, Merc C320 CDI. All are very smooth and swift.
The best diesel engine I've driven is the Audi v6 3 litre. It doesn't sound like a diesel at any speed but is still not as good a sound as the IS250.