OS X wouldn't be able to used the iPhone OS software ecosystem and market (which is the main selling point for this device, both to shareholders, consumers, and publishers). It'd also have to have code changes to run on the low-power ARM chip it uses which provides the 10+ hour battery life.
I think you misunderstand how light its weight is. And 10 inches is pretty small. Too big to fit in your pocket, but probably, what, A4 paper size? Smaller?
Think about carrying a Harry Potter book around with you. A mild annoyance in the sense that you need to carry something at all, but not any kind of muscle strain or burden.