Hmm, good point.
The farthest into double checking that I've been ale to get is the display half of the circuit. I've checked so many times that the RS, EN, and D4-7 pins are connected to the right pins, and they are. For some reason though, when I power the circuit up, I get the first row filled with the black rectangles that make up the characters' spots, and the second row is empty. At the moment, getting the display working is the first priority, because the rest relies on it, and the rest is much easier to test/fix.
I'll see if I can come up with some good photos, but does that problem scream out to anyone as being caused by a particular thing?