Apparently, the main issue with eye strain from close monitors and such is the fact that your eyes have to focus closer. But the oculus has two screens and optics, which they use to let your eyes focus at infinite distance. Hence, eye strain is not an issue.
You'll get sick in the stomach long before any effect on your eyes in any case - having it on for hours on end is extremely unrealistic right now.
There are other subtle effects like fish-eye that are lead to a disoriented feeling when I take off the Oculus.
Think of it this way: your cortex is always learning. Always. So if you present visual input that is subtly different (eye-spacing, fish-eye / pin-cushion, lag) for long periods your brain will learn that. Take it away and you've effectively got mild (hopefully reversible) brain damage wrt the physical world.
There are software issues caused by the game / simulation camera FOV mismatch.
E.g. if you walk up to a wall in the Tuscany demo, it'll look like the wall at the center of your view is slightly closer to you than the wall off-center. It's not such a bad artifact in-game, but when I take the goggles off, flat walls IRL look slightly bowed away from my center of view for a while, and things feel ... different.
It's similar to when you stare at a scrolling screen, and then look at a fixed object. The object seems to be moving in the opposite direction.
