I spent a bit of time looking at the wiring diagram and checking continuities/voltages/etc. trying to figure out how it all works. The diagram showed two optional limit switches in the same circuit as the pressure switch. I found one on the heat exchanger and eventually found the second one on the blower. I too was starting to suspect the circuit board because both limit switches seemed to be okay. When checking voltages, I found that I had only ~9 volts at the pressure switch, but 25 volts at the circuit board. Upon further checking, I found that one of the wires going to the limit switch on the blower had no continuity through it. (It must have had passed some current, but just had very high resistance, because I still got some voltage at the pressure switch.) How a wire goes bad when it's in a dry environment and doesn't move is beyond me, but nevertheless I guess it must have had some corrosion or something build up at one of the connectors or something. A piece of wire and a couple of spade connectors later and my furnace works again. And now I know a lot more about how a furnace works.