This year's SIGGRAPH had an interesting segment covering Physically-based BRDFs, that offered some sobering insights into the problems of traditional realtime lighting models. Particularly, how current models not only produce less believable results, but force artists to waste time and effort compensating for their definciencies. Naturally, they explained how to derive better lighting models that behave more realistically, and offered some insights into the behavior of real-world materials.
Implications for Deferred Shading:
The difference that concerns me most is that the Physically-Based BRDFs rely on Fresnel terms for both their diffuse & specular calculations. Specifically, how these Fresnel terms require the per-light Light Direction & Half Angle vectors. So in a Deferred Renderer, each object's material's Fresnel coefficients must be stored in the G-Buffer so that they can be available for the lighting phase. Also, for materials that use per-RGB Fresnel coefficients, I would need to be able to accumulate colored specular lighting.
These conditions present problems for trying to integrate such a system into a Light Pre-Pass (LPP) renderer. At a minimum, I would need to add single or per-RGB Fresnel coefficients to the already overcrowded G-Buffer. So I really have no choice but to add another render target to the G-Buffer. Then per-RGB fresnel would force me to have a dedicated specular light accumulation buffer, rather than using the approximation trick.
Ultimately, these changes would make the LPP renderer more bloated than a standard Deferred Renderer. So if I wanted to start using a Physically-based BRDF, I would have to make the switch. Now I'm at an impasse, because my initial tests have demonstrated enough of a visual improvement to peak my interest. This has left me with much to consider before I can proceed.