Putting one of the lenses on my MacBook Pro Retina 15" shows the pixels almost as clearly (which makes sense, it's 220 PI). Putting the lens over my iPhone 4S (326 PPI) you could still make out pixels but it looked a lot better.
So given how most the lens magnifies and how close the screen is your eyes, I'd say we'll need screens of 400+ PPI before Oculus-style VR headsets don't distract you with their pixels.
It will be interesting to see what pixel densities are available when the "production" Rift ships.