Apache-like Updates on the way for IIS
One thing that the Application Request Router provides, which appeals to the sysadmin/ops guy in me, that mod_proxy does not do is that you can configure health checks to monitor your back-end servers. This is one of the things presently lacking in mod_jk and mod_proxy. They certainly perform ping tests to make sure that the back-end system is up and running but they can't really tell if a back-end jvm is hung or if there is some other problem preventing the app server from serving up content correctly. Obviously, since I haven't played with it, I'm just reading a feature in a bulleted list of features so this could very well not work as I envision. I'll have to play with it a little to see but it certainly could serve as a replacement for the IIS JK Redirector that the ASF provides for IIS.
Microsoft IIS 7 Implementation and Administration (Mastering)