Same Problem
I have the same problem with the seats in my X3.  They creak when I accelerate or brake.  IT IS DAMN ANNOYING!!!
Took me a while but I have determined that it is one (rear left) seat mount that seems to cause the noise.  I know because I can duplicate the noise by grabbing it and pulling it back and forth.
I've tried lubricating the general area but it doesn't seem to help at all.  I am going to tighten the bolt on the mount tomorrow and if that doesn't work I'm taking it back to the dealer!
Ray