If you watch
MobileTechReview's Sony PRS-T1 review, Lisa (the reviewer) mentions that she had been reading a Kindle 3 instead of the Nook ST or Kobo Touch until she acquired the PRS-T1 because she found the contrast better on the Kindle.
Also, if you watch
EEVblog's Kindle 4 review, the reviewer mentions that the screen on the Kindle 3 appears whiter than on the Kindle 4. You can notice the difference in the video; the Kindle 4's screen appears to have a cooler, blue appearance. I thought this was relevant to mention, considering that screen variations may affect contrast levels, and that supposedly all of these devices have the same screen. Perhaps it's just the matte layer on top that makes the Kindle 4 appear cooler.