From what I have heard; the pricing difference in US will be similar to the pricing difference between 5er and 6er.
2009 X5 is also supposed to get the V8 TT engine and 4.8i name revision.
Notice that X6 has more leather on dash than X5.
From what a birdie said, more goodies like 8-speed gearbox, DCT, air suspension coming later.
There will also be an M-sport package, which may not come to US