According to a report by Car and Driver, Audi will be offering a diesel option on the upcoming 2012 S6 and S7 Sportback models.
Details are nil beyond that, so we have to speculate that the diesel variant will likely have an up-tuned 3.0 liter TDI dialed up to an output mark competitive with the petrol variant. That means less horsepower but more torque for a diesel unit. We're guessing somewhere in the vicinity of 350 PS (257 kW) and over 650 Nm (479 lb-ft) of torque.
On the petrol front we are expecting a new turbocharged 4.0 liter V8 unit, which Audi will share with the Bentley Continental GT, with a power figure north of 435 PS (320 kW / 429 bhp).
The diesel option on those models will likely be popular in Europe but far less so in North America, if Audi even opts to offer it there.