Here's an interesting document that shows charring and ignition temperatures for various woods over specific periods of time. At 250 degrees, ignition is accomplished in a relatively short period of time. While at 180 degrees, ignition takes a bit longer, dependent on the type of wood. Be sure to scroll down until you see the chart at the bottom.
What's even more interesting is that in their tests, the heat source was above the wood samples. In our case with the CS smokers our heat source is below the wood, and in some cases touching the wood, or at least touching the surface that the wood is sitting on.
While our smoking wood might not exactly catch fire, it should definitely be charring at temperatures as low as 180 degrees.