I will give what you said a try. If you add the following to the bottom of vars.php:
$is_IIS5OrLess = (strpos($_SERVER['SERVER_SOFTWARE'], 'Microsoft-IIS/4') !== false && strpos($_SERVER['SERVER_SOFTWARE'], 'Microsoft-IIS/5') !== false) ? true : false;
This will test if it is an unsupported version of IIS. However this can get ugly putting this everywhere. That is why I think maybe as a future goal the WordPress team can make the detection more task based instead of server based.
For instance variables like:
$is_redirectSupported = $is_IIS && $is_IIS5OrLess == false;
Because the servers are becoming more and more similar with the release of IIS 7.0 integrated FastCGI, etc. Plus with IIS 7.0 integrated pipeline more plug-in options are going to be present for WordPress such as integrated Active Directory support. Including many of the features that are currently loved by Apache such as permalinks, url rewrites, and the others you mentioned.
I think the code base would be greatly enhanced by task based support because servers are going to change. There are even problems with WordPress on Apache 1.3 that are not in 2.0. So to treat all the servers the same just clutters up code with meaningless special cases.
But I don't want to get in to a server war. Because when it gets down to it we are trying to make the best blog software possible irregardless of server.
I am hoping to prove you wrong about IIS, because I am going to put my WordPress live in a couple hours running on IIS 6.0 with full URL Rewriting support done via a .NET HttpModule. After it is live I would love to have you guys smash against it and give it a work out.