Some of you have asked me about the dependency between ARR and URL Rewrite. Strictly speaking, there is no hard dependency between ARR and URL Rewrite modules. A more accurate way to describe the relationship between ARR and URL Rewrite is that ARR depends on another module to determine which server group a request should be routed to.
URL Rewrite is one such module that can determine the destination server group based on HTTP and server variable rewrite rules.
I've written a blog about this to further explain how these two modules work and how the same extensibility point in ARR can be used for developers to write their own modules to integrated with ARR. The blog post is at http://blogs.iis.net/wonyoo/archive/2008/07/26/relationship-between-application-request-routing-and-url-rewrite-modules.aspx.