WhoAmI - CRM 4.0

var xmlhttp = new ActiveXObject("Msxml2.XMLHTTP");
xmlhttp.open("POST", "/mscrmservices/2007/CrmService.asmx", false);
xmlhttp.setRequestHeader("Content-Type", "text/xml; charset=utf-8");
xmlhttp.setRequestHeader("SOAPAction", http://schemas.microsoft.com/crm/2007/WebServices/Execute);

var soapBody = ""+
""+
""+
"";

var soapXml = ""xmlns:soap='http://schemas.xmlsoap.org/soap/envelope/' "+
"xmlns:xsi='http://www.w3.org/2001/XMLSchema-instance' "+
"xmlns:xsd='http://www.w3.org/2001/XMLSchema'>";
soapXml += GenerateAuthenticationHeader();
soapXml += soapBody;
soapXml += "";

xmlhttp.send(soapXml);

xmlDoc=new ActiveXObject("Microsoft.XMLDOM");
xmlDoc.async=false;
xmlDoc.loadXML(xmlhttp.responseXML.xml);
var userid = xmlDoc.getElementsByTagName("UserId")[0].childNodes[0].nodeValue;

alert(userid);

Enjoy,
Rami Heleg