I read some of this:
Gasoline FAQ - Part 3 of 4... very interesting.
What I gather out of that is the following: The octane rating is related to how "controlled" the burn is of the measured fuel. Basically, it is a measure of
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the ability of the unburnt end gases to spontaneously ignite under the specified test conditions
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So even though diesel fuel may have a higher auto ignition temperature, that doesn't matter... it's what the diesel fuel does once it has been ignited that matters. If there's a single ignition source (as I believe the Octane Rating test setup specifies) and the molecular structure of the fuel cannot prevent the unburnt end gases from spontaneously igniting, then it gets a lower octane rating. The rating is based on the extremes of behavior between "normal heptane" at the low end and "iso-octane" at the high end.
What Odin mentions makes sense then... once the diesel fuel ignites, it is an "uncontrolled" explosion, and the Diesel engine is designed to handle that... injecting the fuel at the top of the stroke rather than earlier in the compression cycle. On the other hand, the gasoline engine *must* have a controlled explosion, with no secondary wave fronts caused by the preliminary detonation of the unburnt gases... otherwise the engine will eventually destroy itself.