OK, so I ran the math for a few display sizes and resolutions, based on the 3438 x 1/n = formula posted above.
The current 27" iMac already looks to be a retina display, at a viewing distance of only 2.6 feet at least, which seems pretty reasonable for a 27" display. It sports a PPI of 109. But maybe Apple will define the viewing distance as 2 feet or 2.5 feet. Or maybe they'll release a 32" iMac.
At WQHD resolutions - double the current 1080p standard - a 27" monitor would become a "retina" display at just 1.76 feet, with a PPI of 163 (higher than the first laser printers!).
Even at a 32" diagonal screen size, you'd be looking at a 138 PPI display and a "retina" distance of only 2.08 feet. So I could see Apple releasing just such a beast as their next iMac. I doubt anybody is gonna sit closer than 2' to a 32" display (unless they're trying to get a tan...).