The thing is that it should include the view direction as well, since fresnel's law applies as well, when the light is scattering out of the surface to the viewer's direction and not just when light is scattering into the surface.
When the light leaves the surface back into the air, yes some will reflect off the boundary back into the surface, but seeing as it's an infinitely thick plane with zero absorption (and for simplicity, let's say the same IOR as air), it will eventually all make it back out. The only effect that this will have is to bias the 'diffuse' distribution slightly more towards the normal (as at every 'attempt' for light to escape, this direction has the maximal chance). Light is scattering out of the surface in every direction, not just the viewer's direction, so it's not special.
There's still no reason to take the viewer's position into account when measuring the amount of light that refracts into the plane -- where I'm standing relative to a prism has no bearing on the amount of light that's refracted upon striking it's surface.
 d'oh. You need to take the viewers position into account in the exit fresnel equation to measure the amount of diffuse light that is emitted in the direction of the viewer, because that's what I'm measuring. That's pretty damn obvious once I say it out loud... [/edit]
Edited by Hodgman, 01 March 2013 - 06:52 AM.