I think that Gen. Shinseki had an idea that things were going to be a lot rougher than the Administration thought (hoped?), but he was forced into retirement in 2003 shortly after making his views public.
While I'm not going to adopt the Democrat talking-point that the Administration had "no plan" for the occupation, I will say that the plan that they had going in was inadequate and that I don't think that they adapted to the reality of what was going on in Iraq until it was too late to stop the insurgency.