20/20
Hi,
20/20 is the lower limit of the normal visual acuity. People with "normal" vision can have an acuity up to 20/12. The acuity test is not about distinguishing individual pixels: it is about discriminating gap between pixels. Both Touch HD & HTC Desire are slightly subpar to the iPhone 4 (they are 250 dpi).
See this blog for a thorough analysis of this controversy:
http://www.kybervision.com/Blog/files/AppleRetinaDisplay.html
Hope this helps!
Bill