None, I didnt take my preordered one from gamestop when they frist came out because they told me that if you had a defective unit with dead pixels (regardless of what $ony tries to say, dead pixels ARE defects). They tried to tell me it would be "months" before I would find another in stores, I walked out went across the street and walmart had a big old stack of them. I lucked out. 0 pixels on my first try.
At least Nintendo has a policy that a DS with even a SINGLE pixel can be exchanged.