I agree with you.
BMW does not upgrade his US build models in March but in the fall!
As for now the X5 diesel will be introduced as a 09 model.
I was also told that the engines of the X6 would be introduced in the 09 X5 models.
We'll have to be patient........
I would say its both fall and spring. US intros in spring: E60 facelift was introduced in spring 2007. E90 was introduced in spring. X6 was intoduced in spring 2008.......