There is no magical 300 ppi limit as you claim. Retina display is a completely made up marketing term. It has no clear definition, and will be used by Apple to market any device they release in the future that they feel has a high enough resolution.
Apple is about to release an iPad 2 with a resolution of 2048 x 1536 which calculates to a ppi of 260, and they have every intention of marketing the device as having a Retina display.
If a display with a ppi of 260 can be considered Retina just because the device has a larger screen, why can't a device with a ppi of 285 be called Retina since the device will likewise have a larger screen?
The iPad won't be the last time you see the term either. I fully expect to see Macbook Pros, iMacs, and Cinema Displays with substantially higher resolutions in the future, and these devices will also likewise be marketed with a term Retina display, even though most of them will not have a ppi above 300 either, despite the substantial upgrade in resolution. The vast majority of laptops, monitors, hdtvs have a dpi of around a 130 or so. I fully support Apple in them marketing any display that atleast doubles that number as a Retina Display, because that in itself is a mind blowingly impressive achievement.
And if you think the average consumer gives two craps about whether the display is 300 ppi vs. 285 ppi, you are wrong. If you're being reasonable, you shouldn't care about the 15 ppi difference either. If the screen looks sufficiently high res and the pixels sufficiently difficult to see, it doesn't actually matter the precise ppi number.
So again I repeat. Retina display is a completely made up marketing term. It has no clear definition, and it can be used to mean whatever Apple wants it to mean.