One of the things that Salesforce.com is lacking is a descent tree control to display hierarchical data. I did notice a screenshot on the Winter '09 Developer Screencast of a tree control but according to our Salesforce.com technical reps there are no plans to release this.

There is a really good Flex tutorial on developer.force.com concerning this topic, but one of their main takeways is:

NOTE: You should be very careful when using nested queries as you can easily run into governer limits.
We have a large hierarchy with thousands of nodes, so governor limits is certainly a consideration. Therefore, I decided to create a tree that loads the first two levels of the hierarchy and then lazily loads the subsequent level childen when they are clicked on.

You can run this demo on my Developer Site. You might also want to check out this post on the basics of connecting Flex to Salesforce.com.

There is a really good tutorial on Adobe's site that has alot of great information regarding XML and trees if you are interested. I have some plan to add drag and drop capability in the near future.

<?xml version="1.0" encoding="utf-8"?>
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml"
    backgroundGradientAlphas="[1.0, 1.0]"
    backgroundGradientColors="[#F3F3EC, #F3F3EC]"
    creationComplete="login()"
    layout="horizontal"  
    height="300" width="500">

    <mx:Script>
    <![CDATA[
        import com.salesforce.*;
        import com.salesforce.objects.*;
        import com.salesforce.results.*;
        import mx.collections.ArrayCollection;
        import mx.controls.Alert;
        import mx.collections.XMLListCollection;
        import mx.events.ListEvent;

        [Bindable] private var categoriesXml:XML =
	<list>
              <root>
                <level0 name="Categories" level="0"/>
              </root>                       
          </list>;               

        [Bindable] public var sfdc:Connection = new Connection();
        [Bindable] private var acClicked:ArrayCollection = new ArrayCollection();
        [Bindable] private var categoriesXmlData:XMLListCollection = new XMLListCollection(categoriesXml.root);       

        private function login():void {

            var lr:LoginRequest = new LoginRequest();
            lr.server_url = this.parameters.server_url; 
            lr.session_id = this.parameters.session_id;    
            lr.callback = new AsyncResponder(loginSuccess, loginFault);
            sfdc.login(lr);      

        }               

        // gets all level 1
        private function getLevel1():void
        {

            sfdc.query("Select c.Name, c.Id From Cat1__c c Order by c.Name",
                 new AsyncResponder(
                    function (qr:QueryResult):void {
                        var parent:XMLList = categoriesXml.root.level0.(@name == 'Categories');
                        if (qr.size > 0) {
                            for (var j:int=0;j<qr.size;j++) {
                                var newNode:XML = <level1 level="1"/>;
                                newNode.@id = qr.records[j].Id;
                                newNode.@name = qr.records[j].Name;
                                parent[0].appendChild(newNode);
                                getLevel2(newNode.@id);
                            }
                        }
                    },sfdcFailure
                )
            );  

        }     

        // gets all level 2 nodes for the selected parent
        private function getLevel2(parentId:String):void
        {

            sfdc.query("Select c.Id, c.Name From Cat2__c c Where c.Cat1__c = '"+parentId+"' Order By c.Name",
                 new AsyncResponder(
                    function (qr:QueryResult):void {
                        var parent:XMLList = categoriesXml.root.level0.level1.(@id == parentId);
                        if (qr.size > 0) {
                            for (var j:int=0;j<qr.size;j++) {
                                var newNode:XML = <level2 level="2"/>;
                                newNode.@id = qr.records[j].Id;
                                newNode.@name = qr.records[j].Name;
                                parent[0].appendChild(newNode);
                            }
                        }
                    },sfdcFailure
                )
            );  

        }         

        // gets all level 3 nodes for the selected parent
        private function getLevel3(parentId:String):void
        {

            sfdc.query("Select c.Id, c.Name From Cat3__c c Where Cat2__c = '"+parentId+"' Order By c.Name",
                 new AsyncResponder(
                    function (qr:QueryResult):void {
                        var parent:XMLList = categoriesXml.root.level0.level1.level2.(@id == parentId);
                        if (qr.size > 0) {
                            for (var j:int=0;j<qr.size;j++) {
                                var newNode:XML = <level3 level="3"/>;
                                newNode.@id = qr.records[j].Id;
                                newNode.@name = qr.records[j].Name;
                                parent[0].appendChild(newNode);
                            }
                        }
                    },sfdcFailure
                )
            );  

        }                               

        // queries sfdc when node is clicked
        private function tree_itemClick(evt:ListEvent):void {

            var item:Object = Tree(evt.currentTarget).selectedItem;
            var node:XML = XML(item);
            var nodeName:String = node.@name;

            if (!acClicked.contains(nodeName)) {

                   // fetch the children
                getLevel3(node.@id);

                // add the node to the list of nodes clicked          
                acClicked.addItem(nodeName);

            }

        } 

        private function loginSuccess(result:Object):void {
            getLevel1();
        }                               

        private function sfdcFailure(fault:Object):void {
            Alert.show(fault.faultstring);
        }  

        private function loginFault(fault:Object):void
        {
            Alert.show("Could not log into SFDC: "+fault.fault.faultString,"Login Error");
        }          

    ]]>
    </mx:Script>

    <mx:Tree id="tree"
        dataProvider="{categoriesXmlData}"
        itemClick="tree_itemClick(event)"
        labelField="@name"
        showRoot="false"
        width="100%" height="100%" left="0" top="0"/>

</mx:Application>