The Autoignition Temperature of a standard unleaded gasoline can be anywhere from 260 to 460 degrees C (or 500 to 860 deg F) as quoted on the FAQ: Automotive Gasoline Web Page by Bruce Hamilton (this page is also an excellent and accurate resource about the science of gasoline)
So as you can see the temperature of the cigarette, even at the side of the lit portion, is more than enough to cause gasoline to autoignite. However, there are many other factors that one should take into account. It matters how the cigarette actually falls onto the surface to the gasoline. There is a lower chance of autoignition if the cigarette falls on it's side where the temperature is lower. Also, the temperature of the gasoline itself matters. If the gasoline is cold to start with then there is again a lower chance of autoignition. One should also consider the amount of the gasoline that you have, namely if you have a large volume of gasoline that would mean that the there is enough surrounding liquid for the heat to go into and therefore the temperature of the gasoline-cigar contact spot would due to heat conduction of the gasoline decrease rapidly, therefore reducing the chance for autoignition. On the contrary, if you have a nice thin film of the gasoline, the chances of the autoignition increase. Also, the evaporation of the gasoline at the point of the contact will also act to reduce the actual contact temperature rapidly.
We are aware of at least one experimental trial (conducted by our colleague at the university) where the gasoline did not ignite upon contact with a lit cigar. This just means that all of the above conditions were not in favour of the ignition.
However, It is important to realize that the gasoline vapour has a much lower autoignition temperature than the gasoline itself. Namely, if you spill gasoline on a hot road (say in the hot summer day) you will be able to ignite gasoline by contact with a cigarette easily, just because of the gasoline vapour layer that would be produced above the surface of the gasoline. Not to even mention throwing the cigarette into the container with gasoline that has been closed for some time and is therefore full of gasoline vapours.
So for all of you smokers out there that are wondering why you are not allowed to smoke at gas pump stations, these are the real scientific reasons. It is dangerous and science is telling us that the temperature of the cigarette, given the appropriate conditions, is enough to cause gasoline to autoignite (and in case of the gas pump station this would be disastrous.)
So, Mike it looks like John Grisham has done his homework before writing his book.WARNING: All above mentioned experiments were performed by professionals under controlled conditions and with proper precautions. You are in no way to attempt this on your own. This is dangerous and you can cause serious injuries to yourself and others. PhysLink and its authors will not be held responsible.