The IE team has made a very wise decision to natively support XMLHttpRequest in IE 7. XMLHTTPRequest is the foundation of any AJAX usage, and I for one applaud the move to make it available without the demand for using ActiveXObject.

Using object detection, one can easily make your code backwards compatible as well:

var oXMLHttp;
if(typeof XMLHttpRequest != "undefined"){
	/* Code for: 
		IE 7
		Firefox, Mozilla etc
		Safari
		Opera
	*/
	oXMLHttp = new XMLHttpRequest();
}
else if(typeof window.ActiveXObject != "undefined"){
	/*
		Code for:
			IE 5
			IE 6
	*/
	oXMLHttp = new ActiveXObject("Msxml2.XMLHTTP");
}