As far as i'm aware PSD was never supposed to be a portable format. I believe Adobe did release some limited information on the format in the past and that's why there's a lot of apps that can read and write the format.

It's a complicated format but that i imagine is from the fact that it's highly backwards compatable. Not always with 100% success but i believe it's possible to open new PSD files in older versions of photoshop. It's rare to see compatability work both ways like that.