Moustafa has put together a fantastic sample application that shows how you can use the Rules Engine in Windows Workflow Foundation to execute rulesets against any target object.  In this example, the ruleset prepopulates fields in a Windows Form, validates entries as well as performs calculations.  This example shows the application obtaining the ruleset from an external database, allowing an easy way to separate the logic expressed in the ruleset from the compiled code that makes up the form.  In this way you can alter the specifics of the business logic of your application, say a pricing policy, without diving into the code of the application.

If you're interested in learning more about the Rules Engine, these other samples are available on the community site.

Also, Jurgen has written an excellent article on MSDN discussing the Rules Engine.