A draft

/*
/* SharePoint JavaScript based cascaded dropdowns
/* Published on spcd.codeplex.com
/* Visit www.sharepointboris.net for more SharePoint tips and Tricks
/* Tested on SharePoint 3.0 / 2007
*/

var setupComplete = false;
var wssSvcUrl = L_Menu_BaseUrl + "/_vti_bin/lists.asmx";
//Initiate function
function addHandler() {
for (i = 0; i < CascadingDropdowns.length; i++) {
if(CascadingDropdowns[i].parentLookup.isDropDown) {
CascadingDropdowns[i].parentLookup.Object.onchange = function() { FilterChooicesForMyChild(this); }
}
else {
CascadingDropdowns[i].parentLookup.Opthid.onpropertychange = function() { FilterChooicesForMyChild(this); }
}
}
setupComplete = true;
}

//support functions
function getField(fieldType,fieldTitle) {
var docTags = document.getElementsByTagName(fieldType);
for (var i=0; i < docTags.length; i++) {
if (docTags[i].title == fieldTitle) {
return docTags[i];
}
}
return false;
}

//Object for working with web services
function WssSvcCall() {
this.soapQuery = "";
this.url = "";
this.returnFunctionName = function() { return; };

this.Submit = function() {
http_request = false;
if (window.XMLHttpRequest) { // Mozilla, Safari,...
http_request = new XMLHttpRequest();
if (http_request.overrideMimeType) {
// set type accordingly to anticipated content type
//http_request.overrideMimeType('text/xml');
http_request.overrideMimeType('text/html');
}
} else if (window.ActiveXObject) { // IE
try {
http_request = new ActiveXObject("Msxml2.XMLHTTP");
} catch (e) {
try {
http_request = new ActiveXObject("Microsoft.XMLHTTP");
} catch (e) {}
}
}
if (!http_request) {
alert('Cannot create XMLHTTP instance');
return false;
}

http_request.onreadystatechange = this.returnFunctionName;
http_request.open('POST', this.url, true);
http_request.setRequestHeader("Content-type", "application/soap+xml");
http_request.setRequestHeader("Content-length", this.soapQuery.length);
http_request.send(this.soapQuery);
}
}

//Object to hold the lookup field and needed properties
function LookupField(LookupFieldTitle) {
this.Object = false;
this.Opthid = false;
this.isDropDown = true;

if(getField('select',LookupFieldTitle))
{
//if lookup has 19 or less items - SELECT
this.Object = getField('select',LookupFieldTitle);
}
else
{
//if it has 20 or more items - INPUT
this.Object = getField('input',LookupFieldTitle);
this.Opthid = document.getElementById(this.Object.optHid);
this.isDropDown = false;
}
}

var CascadingDropdowns = new Array();
//Object to hold cascading relationship info
function cascadeDropdowns(ParentDropDownTitle, ChildDropDownTitle, Child2ParentFieldIntName, ChildListNameOrGuid, ChildLookupTargetField) {
this.parentLookup = new LookupField(ParentDropDownTitle);
this.childLookup = new LookupField(ChildDropDownTitle);
this.childList = ChildListNameOrGuid;
this.child2ParentLink = Child2ParentFieldIntName;
this.childLookupTargetField = ChildLookupTargetField;

CascadingDropdowns.push(this);
}

function FilterChooicesForMyChild(triggerObject) {
if(!setupComplete) return; //security in IE not to trigger filter on load
for(i = 0; i < CascadingDropdowns.length; i++) {
if(CascadingDropdowns[i].parentLookup.Object == triggerObject || CascadingDropdowns[i].parentLookup.Opthid == triggerObject) {
var CascadingDropdown = CascadingDropdowns[i];
var wssSvc = new WssSvcCall();
wssSvc.soapQuery = getQuery2Run(CascadingDropdown);
wssSvc.url = wssSvcUrl;
wssSvc.returnFunctionName = function() { filterChildLookup(CascadingDropdown); }
wssSvc.Submit();
}
}
}

function getQuery2Run(CascadingDropdown) {
var selectedId;
if(CascadingDropdown.parentLookup.isDropDown) {
selectedId = CascadingDropdown.parentLookup.Object.options[CascadingDropdown.parentLookup.Object.selectedIndex].value;
}
else {
selectedId = CascadingDropdown.parentLookup.Opthid.value;
}
var result = '' +
'' +
'' +
'' +
'' + CascadingDropdown.childList + '' +
'' +
'' +
(selectedId==0?'':'' + selectedId + '') +
'' +
'' +
'' +
'' +
'' +
'' +
'' +
'' +
''
return result;
}

function filterChildLookup(CascadingDropdown) {
if (http_request.readyState == 4) {
if (http_request.status == 200) {
var xmlResult = parseXML(http_request.responseText);
var resultNodes = xmlResult.getElementsByTagName('z:row');
if(CascadingDropdown.childLookup.isDropDown) {
var startAt = 0;
if(CascadingDropdown.childLookup.Object.options[0].value == 0) startAt = 1;
CascadingDropdown.childLookup.Object.options.length = startAt;
for(y = 0; y < resultNodes.length; y++) {
CascadingDropdown.childLookup.Object.options[y + startAt] = new Option(attributeValue(resultNodes[y], "ows_" + CascadingDropdown.childLookupTargetField), attributeValue(resultNodes[y], "ows_ID"), false, false);
if(CascadingDropdown.childLookup.Object.options.length > 0) CascadingDropdown.childLookup.Object.options[0].selected = "selected";
}
}
else {
var choices = CascadingDropdown.childLookup.Object.choices;
if(choices.substr(choices.indexOf("|"),3) == "|0|") choices = choices.substr(0, choices.indexOf("|")+2);
var choicesArr = new Array();
for(y = 0; y < resultNodes.length; y++) {
choicesArr.push(attributeValue(resultNodes[y], "ows_" + CascadingDropdown.childLookupTargetField) + "|" + attributeValue(resultNodes[y], "ows_ID"));
}
choices += (choicesArr.length==0?"":"|") + choicesArr.join('|');
CascadingDropdown.childLookup.Object.choices = choices;
CascadingDropdown.childLookup.Object.value = "";
CascadingDropdown.childLookup.Opthid.value = "";
}
} else {
alert('There was a problem with the request.');
}
}
}

//xml parsing support functions
function parseXML(inputString) {
if (window.DOMParser) {
parser=new DOMParser();
xmlDoc=parser.parseFromString(inputString,"text/xml");
}
else { // Internet Explorer
xmlDoc=new ActiveXObject("Microsoft.XMLDOM");
xmlDoc.async="false";
xmlDoc.loadXML(inputString);
}
return xmlDoc;
}

function attributeValue(node, attributeName) {
var attributesCollection = node.attributes;
for(atv = 0; atv < attributesCollection.length; atv++) {
if(attributesCollection[atv].name == attributeName) return attributesCollection[atv].value
}
return "";
}

//Start the party after the DOM is loaded
_spBodyOnLoadFunctionNames.push('addHandler');