I built my own ICF house in the Winter of 06/07.  There is one thing I am kicking myself about.  I did not put insulation under my basement slab.  (I was not well versed on energy efficiency and the salesman didn't push it hard enough.)  Now I have a dilemma.  What is the best way to insulate my floor with as little thickness as possible? 
Do I use P2000 under carpet and padding?
Can I use Insultarp under the carpet and padding?
Is the reflective bubble wrap better?
Can I put down P2000 or Insultarp and overlay with Permacrete?

Right now I am not worried which one costs more - even though I am on a somewhat strict budget, I just want the most effective method of insulating by a thickness of no greater than an 1 1/2".
Thanks.

Is the floor flat?

Consider the old method of strapping the floor @ 24" o.c. with P.T. 2x materials and insulate with 1 1/2" EPS panels in between

Yes, the floor is flat. The problem with 1 1/2" EPS is that my doors and trim are all set, so I don't quite have that much room. (By the time you get the foam, then the pad, then the carpet, I would have 2")

So just cut your door bottoms & jambs, you'll want to pull off and reset your trim anyways if your raising the floor height. I don't know why people get so set of loosing a couple inches of height, unless your already under the 7ft mark. In the end of the day, most will never notice it.

Also, why even use the PT sleepers? Run the XPS full and just overlay with T&G plywood or OSB for your subfloor. no more thermal bridge.