I just published a free bi-directional (inbound & outbound) URL re-writing module for IIS named Transformer.NET available for download at http://www.gdssecurity.com/l/t.php.  The current Beta version provides support for regex-based URL re-writing, allowing URLs in both incoming HTTP requests and outbound HTTP responses to be re-written on-the-fly. This flexibility gives you the ability to alter website URLs without the need to modify any underlying website/application source code (i.e. great for DotNetNuke).

I am using it successfully on a few sites that I run to make the Forum url's super short, extension-less, and very human (and search engine) friendly.  It also supports Rrewrite Maps (like Apache's mod_rewrite) so you can do key/value substitutions (used in the examples below to include the forum "name" within the url instead of the "id"). 

Here's an example of the before & after from various DNN forum urls:

Before: http://localhost/Forum/tabid/55/forumid/3/scope/threads/Default.aspx 
After:  http://localhost/forums/support

Before: http://localhost/Forum/tabid/55/forumid/3/threadid/2/scope/posts/Default.aspx
After:  http://localhost/forums/support/thread/2

Before: /Forum/tabid/55/ctl/PostEdit/forumid/3/postid/2/action/reply/mid378/Default.aspx
After: http://localhost/forums/support/2/378/reply 

There's a blog post that explains more about it at http://www.gdssecurity.com/l/b/2008/02/27/bi-directional-http-transformation/, and the download includes a detailed user guide / documentation.  If anyone is interested, I can also post the rewrite rule set I use for generating forum url's as shown above.