Most of the technically-oriented content on the subject of inference engines is on the Development site.
Updated Oct 20 2008:
There were some interesting responses to an article at the athico.com blog site about rule engine performance.
A comment by Greg Barton outlines a disaster story about the inappropriate use of a rule engine to implement transaction processing for a large telecom application.
The article is devoted to Windows Workflow Foundation ( WF ) and the Microsoft Business Rule Engine (MS BRE), but it is a good introduction to the workings of almost any inference/rule engine.