Current V8 diesel is not sporty oriented enough. We'll have to wait till NGV8D - coming to power F01 7er, X6 & F5. Etc. BMW are very strict when it comes to driving dynamics: big diesels are not the way to go.

Also V8 diesel models sales (7er) are very tiny. It's more about image.
Also almost no demand for V8 diesel in (new) X5. Thus BMW are considering not to offer it at all. I6 VTT engine is much more appreciated & demanded, and it will be offered soon.

Mind that Europe is main diesel market, and here big diesels do not sale well. Perhaps when diesel models enter US market with 2008 we'll then soon seen models bigger diesels too.

What BMW strives to is EfficientDynamics. While V8, V10 or even V12 diesel still sound like oxymoron.