It will be interesting to see if some of the fonts on the IPAD are too small and cause more eye strain. ON an EYE CHART you know how it goes from BIG LETTERS to smaller and smaller. That could be factor, your ereader has all the fonts the same size, how about putting a smaller font on it and seeing how reading is after 30 minutes.
I know if I read something that is small , it causes eye strain. Also if my eyes are tired, looking into bright light can cause eye strain too. Many factors , it will be interesting to see if research is done on the IPAD and vision problems.
p.s. The day the IPAD came out I went to BEST BUY to try it out for a few minutes and then I tried 3D TV with 3D glasses. I did experience eye strain after using those glasses for 2 or 3 minutes. Walked out of the store with slight headache.