It seems the function which defines XMLHttpRequest for IE has regressed and does not work as expected in all environments. Before 1.1.1 (I have not checked all versions but at least in 1.02) the following implementation was used:

XMLHttpRequest = function(){
		return new ActiveXObject(
			navigator.userAgent.indexOf("MSIE 5") >= 0 ?
			"Microsoft.XMLHTTP" : "Msxml2.XMLHTTP"
		);
	};

The implementation in 1.1.1 is simplified (IMHO too much):

XMLHttpRequest = function(){
		return new ActiveXObject("Microsoft.XMLHTTP");
	};

A bug occurs using XMLHTTPRequest in IE 5.5 or 6 (I have http://tredosoft.com/Multiple_IEMultipleIE installed).

IE always uses POST in this case, which is only noticable if e.g. a the Ajax servlet implements GET only or uses a different implementation for GET and POST. So quite hard to track, I only found the problem using http://www.fiddlertool.com which tracks all HTTP traffic.

A proposed solution is:

XMLHttpRequest = function(){
		// version depends on ms msversion NOT browser version
		// wrong version sends always POST!
		try {
			return new ActiveXObject("Msxml2.XMLHTTP");
		} catch (e) {
			return new ActiveXObject("Microsoft.XMLHTTP");
			} 

Maybe the original implementor would like to look into that?