[jfrancis]

1) How would you do a dusty window front lit?

I'd probably map the dust on over glass as some sort of diffuse, warm gray near-Lambert surface through a layer mask.

2) How would you define the same surface so as to look good when back-lit?

Here I'm a little less sure. I'd probably look into thin SSS or maybe roughness on the glass, although I think roughness is probably the worse idea and would end up looking like frosted glass. So thin SSS, I guess. for the dust.

3) But here's my main question: Shouldn't there be a single shader that looks good either front-lit or back-lit? Or do there really need to be two qualitatively different approaches for the two scenarios? And if so, is this a case where you'd use the 'back-face materials' functionality? Or can you think of a solution in which one approach would look good in either lighting case?