The JavaRules blog has done an interesting take on the rise of the Open Source BRMS, titled "Drools, BRMS and Revolutions".
Here is a choice quote:
"Here’s another thing: More and more companies are using shareware or freeware products such as Linux, Open Office, Java, Eclipse, Net Beans, JBoss to run those $500K (final installed price) BRMS packages. Now they’re going to have another really good package to add in there; Drools. The line between commercial BRMS vendors and freeware vendors is becoming more and more blurred and, with the next year or two, should disappear altogether."