The IE team and MSXML team and also other client side oriented frameworks(see jQuery) are using a MSXML lookup for the ajax XHR transport object. Wicket 1.4.x versions are using the legacy new ActiveXObject("Microsoft.XMLHTTP") object. Some browsers may disable the object and may cause issues for ajax requests.
I guess using a simple lookup for the MSXML object. The standard seems to be: Msxml2.XMLHTTP but, you may also lookup Msxml2.XMLHTTP.6.0 for IE8 and above.
Using the right XHR object may add security and performance fixes provided by IE. The legacy Microsoft.XMLHTTP object is not recommended.