FWIW, I read a few "accelerator lag" posts before getting my X335i and was paying particular attention to that yesterday on my virgin drives. I did not experience the lag, in fact it is incredibly sensitive compared to the 3 Series I just traded in on it. My wife kept asking why I was "taking off so fast" at lights.
My X3 was built exactly a week ago today (April 3), so perhaps they have updated the software at the factory. If that is the case, a service bulletin should be out soon for dealers.