Nick Malik published a thought-provoking article on rules engines recently. You can read it at http://blogs.msdn.com/nickmalik/archive/2007/03/06/perhaps-it-is-time-to-declare-victory-in-the-battle-of-rules-engines-vs-dependency-injection.aspx. He adopts a fairly negative attitude to rules engines in general, suggesting various reasons why he considers that they are not often useful. His preferred route is to use dependency injection patterns, implementing rule logic in code components that are then loaded in some dynamic or configurable fashion into consuming applications.
I believe Nick is quite wrong to contrast rules engines and dependency injection in this way. In reality, many modern rules engines provide the ideal mechanism for implementing and managing dependency injection patterns. You can read my response at http://geekswithblogs.net/cyoung/articles/108411.aspx.
Previously posted at http://blog.solidsoft.com/blogs/charles_young_mvp_biztalk/default.aspx