• ID: The unique id value of the entity’s object
• ENTITY: The name of the entity to lookup
• IDFIELD: The entity’s primary id field name, like accountid, incidentid, etc.
• LOOKUPFIELD: The name of the field you want to lookup, like customerid
• RETURNTYPE: The name of the entity’s return type, like contact or account. If there are multiple possible, then specify “multiple” and it will pass back the type instead of type name. If you specify “value” it will return just the value.

function getEntityLookup(id, entity, idfield, lookupfield, returntype) {
    var xml = "" +
	"< ?xml version='1.0' encoding='utf-8'?>" +
	"<soap :Envelope 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'>" +
	GenerateAuthenticationHeader() +
	"</soap><soap :Body>" +
	"<fetch xmlns='http://schemas.microsoft.com/crm/2007/WebServices'>" +
	"<fetchxml>" +
	" &lt;fetch mapping='logical' count='1'&gt;" +
	" &lt;entity name='" + entity + "'&gt;" +
	" &lt;attribute name='" + lookupfield + "' /&gt;" +
	" &lt;filter&gt;" +
	" &lt;condition attribute='" + idfield + "' operator='eq' value='" + id + "' /&gt;" +
	" &lt;/filter&gt;" +
	" &lt;/entity&gt;" +
	" &lt;/fetch&gt;" +
	"</fetchxml>" +
	"</fetch>" +
	"</soap>" +
	"";

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

    var resultXml = xmlHttpRequest.responseXML;
    var resultSet = resultXml.text;
    resultSet.replace('&lt;', '< ');
    resultSet.replace('&gt;', '>');

    var oXmlDoc = new ActiveXObject("Microsoft.XMLDOM");
    oXmlDoc.async = false;
    oXmlDoc.loadXML(resultSet);

    var result = oXmlDoc.getElementsByTagName(lookupfield);
    if (returntype == 'value') return result[0].text;

    var lookupItem = new Object();
    for (var i = 0; i < result[0].attributes.length; i++) {
        var att = result[0].attributes[i];
        if (returntype == 'multiple' && att.name == "type") lookupItem.type = att.value;
        if (att.name == "name") lookupItem.name = att.value;
    }
    if (returntype != 'multiple') lookupItem.typename = returntype;

    lookupItem.id = result[0].text;

    return lookupItem;
}