I do not believe the US spec'd X3 will get a diesel engine. I believe the 2006 X3 will have the same engines as the 3 series, mainly the 215 hp 3.0 for the 2.5 and the 255 hp 3.0 littre for the 3.0. Don't know if they will change the name of the 2.5 (the base 3 series is still the 325i even though it has a 3.0 litre engine).
I drove the (X3) 3.0d. It is a very nice engine, diesel fuel here in Germany is much cheaper than premium unleaded (ROS 98, super plus unleaded) 1,01 euro per litre for diesel vs 1,27 euro/litre. Very torquey engine. Has comparable performance to the 3.0i (gas engine)
JP