I would presume that IIS is configured to be as restrictive as possible about the string, meaning anything it doesn't recognize as a "default" InternetExplorer string will cause the browser to be rejected.

Actually, I suspect the problem is in the javascript, not in IIS. For the one site I tried, I looked at the javascript that renders it. The problem is that it invokes three other javascripts that I canot get to (easily, anyway...I don't know where they're stored, nor do I know how to get them anyway.) I suspec that theoffending check is in one or more of these javascripts. (My experience in the past with hostile javastripts has been that the javascript itself contains the check for orthodoxy, and that IIS doesn't sully its hands with such stuff. YMMV, of course.)