The main place that pixels were noticeable was in an Apple store. Next to the iPhone 4, the older iPads looked much less sharp, at least for text (particularly Computer Modern). There have been lots of attempts at resolution independence, and I'm sure Windows and various Linux GUIs have hooks for such, but they've not been tested much because the hardware was generally in the 96-120 ppi range. There's no need for HDTVs to change.