I've also been very impressed by the Drools project in general.
I wouldn't see Drools as 'program-in-XML-instead-of-Java' - the underlying approach is very different: With Drools you state what you know to be true , with Java you say exactly what you want done. The Drools approach can lead to more simple code (none of those messy , tangled , if..then statements).
In an 'ideal world' not only could business analysts write rules (if they wanted to) , but other Analysts could use RAD Tools to develop the user interface (the market Visual Basic aimed at). The productivity gain comes from not waiting for a developer to translate requirements into code , with the possibility of making mistakes.
In reality , there will always be requirements that need a Developer (such as writing rules against a Database). As ever , the balance of work between Developer and Analyst will depend on project, organisation and skillsets.