Pixel shaders are just code. You can have them do whatever you want. If you want them to raytrace, they can. If you want them to do something else, they can do that too.
Going back to this- from my (limited of course!) experience of shaders, they are very very simplistic code. you *can't* do anything you want with them. they allow you a basic set of features with which you can get some impressive results. but you tell me, is there something i'm missing? Raytraced shader?
I can make them reference a cube texture though X-)