Originally Posted by Pennhaven
Man I certainly cannot detect any pixelation of text at 150 ppi.
Right now I'm writing using my 24" 1920 x 1080 desktop monitor with a pixel density of about 93 ppi and text seems very sharp to me at 26" distance, not at all fatiguing to look at for hours. To notice the pixels in the text I have to get within 16" inches of the screen. In comparison 150 ppi on my Asus Eee Pad looks extremely sharp. I literally cannot detect any pixelation at the closest my eyes I can focus which is 12" due to presbyopia. Without magnification, it would be tough, if not impossible, for me to distinguish anything sharper.
20/20 vision corresponds to about 143 pixels per inch when viewed from 2 feet, so unless you've got very good vision, you'll probably not need anything more. Of course, for text, your computer is also using subpixel AA, so in practice the actual resolution for text is significantly better then the PPI would imply.
For a tablet it really just depends how far you want to hold it. At 18", anything above 190 PPI isn't helping, and at 1 foot, 286 PPI. Of course, with subpixel rendering, much lower text will likely look just as good.