Firstly, great product guys!!
Secondly, I notice that the code in wp-settings.php dealing with IIS redirection doesn't work for IIS Mod-Rewrite on IIS 6.0.
WordPress only uses the "HTTP_X_ORIGINAL_URL" header if the "REQUEST_URI" server variable is empty. The problem is that, after a rewrite, both of these values are populated ("REQUEST_URI" contains the new rewritten URL eg. "/blog/index.php").
The providers of IIS Mod-Rewrite are providing code hacks to make it work:
Would this be worth fixing in a future version of WordPress?
Being strongly against code hacks we've built a quick plugin that uses "HTTP_X_ORIGINAL_URL" when it exists so no rush on our account. ;)