When you change it to something else (I did to Android 2.3, WebOS) it worked. When accessing with a WP7 user agent string ("Mozilla/5.0 (compatible; MSIE 9.0; Windows Phone OS 7.5; Trident/5.0; IEMobile/9.0; SAMSUNG; SGH-i917)" I was redirected to www.google.com.
Looks like an innocent mistake, however. Or they decided they wouldn't support IEMobile 9.