Unfortunately, I didn't take any before pics, but here is one of the guitar post-cleaning:
The lighter area is legitimate arm wear, not a result of my cleaning efforts. But you can see the difference in the finish there where it was continuously worn down (through the sunburst) vs. the rest of the top, which has a very thin layer of what could be mistaken for dirt. I pretty much left this layer alone -- I just wiped it gently with a damp rag. The thicker areas of buildup that I did approach more <ahem> vigorously were mostly on the edges and looked and felt to me like that buildup of sweat and oil that you get on the fingerboard after a while, but it did have a kind of checked texture to it like aged nitro. But it was also kinda nasty.
Any thoughts about where the line is between patina and dirt?
I'll never look at patina the same way again. 
