heres an article that explains it a bit
On the other hand, the average viewing distance for living room HDTVs in America is around 7 to 10 feet, depending on the screen size. So to appear “perfectly” sharp with 20/20 Vision like the iPhone 4 Retina Display, HDTVs only need a proportionally much lower PPI in order to achieve “Retina Display” status and have the HDTV appear “perfectly” sharp and at the visual acuity limit of your eyes.
Existing 40 inch 1920x1080 HDTV is a “Retina Display” when viewed from 5.2 feet or more
Existing 50 inch 1920x1080 HDTV is a “Retina Display” when viewed from 6.5 feet or more
Existing 60 inch 1920x1080 HDTV is a “Retina Display” when viewed from 7.8 feet or more
Since the typical HDTV viewing distances are larger than the minimum distances listed above, the HDTVs appear “perfectly” sharp and at the visual acuity limit of your eyes. At the viewing distances listed above the pixels on a 1920x1080 HDTV will not be visible by a person with 20/20 Vision in exactly the same way as the Retina Displays on the iPhone 4, new iPad 3, and MacBook Pro at their viewing distances. So existing 1920x1080 HDTVs are “Retina Displays” in exactly the same way as the existing Apple Retina Display products. If the HDTVs had a higher PPI or a higher pixel resolution your eyes wouldn’t be able to see the difference at their proper viewing distances. So existing 1920x1080 HDTVs are already equivalent to what Apple calls a “Retina Display.”
Why Your HDTV Is Already A Retina Display