with the rules capability in the windows workflow foundation [1], parts of business and application logic can be represented by more (policies) or less (conditioned activities) complex rulesets. the most important advantage is, that you can load and update those rulesets dynamically at runtime. e.g., validation logic, calculation, etc. could be sourced out of the application.

to get in touch with the rules engine, read the comprehensive introductory article at [2] and have a look at the corresponding hands on lab [3].
[4] shows how to host the ruleset designer in a custom application, for defining and modifying rules at runtime.

[1] windows workflow foundation
[2] Introduction to the Windows Workflow Foundation Rules Engine
[3] WWF Beta 2 Hands on Labs
[4] external ruleset toolkit