These iPad/Retina Display threads are crazy.
When iTunes starts selling 1080 films, then the resolution may go up. Until then, what's the reason? The current resolution is as it is because iTunes sell 720 films, because YouTube has 720 videos, because the iPhone shoots 720 videos.
For Apple at the moment, HD = 720, except when its convenient for them to remember 1080, such as in the high-end iMac, so they have a new feature to sell.
"Here's your 1080 display we'll market as "full HD". We don't have HD DVD, we don't have BluRay, but have the full HD screen anyway."
Sure, I'm sure the fonts and icons look smoother with the higher resolution, and I'm sure some consumers who edit 1080 videos will be happy, but marketing it with the words "full HD" is a bit of a gimmick when people immediately link "full HD" to watching 1080 films, which they can't do with the iMac's optical drive nor iTunes.