Full code after the jump.

View MXML

<?xml version="1.0" encoding="utf-8"?>
<!-- http://blog.flexexamples.com/2008/05/11/creating-an-editable-datagrid-control-in-flex/ -->
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml"
        layout="vertical"
        verticalAlign="middle"
        backgroundColor="white">

    <mx:ArrayCollection id="arrColl">
        <mx:source>
            <mx:Array>
                <mx:Object label="Student A" score="85" />
                <mx:Object label="Student B" score="48" />
                <mx:Object label="Student C" score="71" />
                <mx:Object label="Student D" score="88" />
                <mx:Object label="Student E" score="24" />
                <mx:Object label="Student F" score="64" />
                <mx:Object label="Student G" score="76" />
                <mx:Object label="Student H" score="76" />
                <mx:Object label="Student I" score="93" />
                <mx:Object label="Student J" score="88" />
                <mx:Object label="Student K" score="48" />
                <mx:Object label="Student L" score="76" />
            </mx:Array>
        </mx:source>
    </mx:ArrayCollection>

    <mx:DataGrid id="dataGrid"
            dataProvider="{arrColl}"
            editable="true"
            rowCount="8">
        <mx:columns>
            <mx:DataGridColumn dataField="label"
                    editable="false" />
            <mx:DataGridColumn dataField="score"
                    editable="true" />
        </mx:columns>
    </mx:DataGrid>

</mx:Application>

View source is enabled in the following example.

When working with editable DataGrid controls, there are three events which can be useful: itemEditBeginning, itemEditBegin, and itemEditEnd.

View MXML

<?xml version="1.0" encoding="utf-8"?>
<!-- http://blog.flexexamples.com/2008/05/11/creating-an-editable-datagrid-control-in-flex/ -->
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml"
        layout="vertical"
        verticalAlign="middle"
        backgroundColor="white">

    <mx:Script>
        <![CDATA[
            import mx.events.DataGridEvent;

            private function addEvent(evt:DataGridEvent):void {
                eventsArrColl.addItem(evt);
            }

            private function clearEvents():void {
                eventsArrColl = new ArrayCollection();
            }
        ]]>
    </mx:Script>

    <mx:ArrayCollection id="arrColl">
        <mx:source>
            <mx:Array>
                <mx:Object label="Student A" score="85" />
                <mx:Object label="Student B" score="48" />
                <mx:Object label="Student C" score="71" />
                <mx:Object label="Student D" score="88" />
                <mx:Object label="Student E" score="24" />
                <mx:Object label="Student F" score="64" />
                <mx:Object label="Student G" score="76" />
                <mx:Object label="Student H" score="76" />
                <mx:Object label="Student I" score="93" />
                <mx:Object label="Student J" score="88" />
                <mx:Object label="Student K" score="48" />
                <mx:Object label="Student L" score="76" />
            </mx:Array>
        </mx:source>
    </mx:ArrayCollection>

    <mx:ArrayCollection id="eventsArrColl" />

    <mx:ApplicationControlBar dock="true">
        <mx:Button label="Clear DataGrid" click="clearEvents();" />
    </mx:ApplicationControlBar>

    <mx:DataGrid id="dataGrid"
            dataProvider="{arrColl}"
            editable="true"
            rowCount="8"
            itemEditBegin="addEvent(event);"
            itemEditBeginning="addEvent(event);"
            itemEditEnd="addEvent(event);"
            itemFocusIn="//addEvent(event);"
            itemFocusOut="//addEvent(event);">
        <mx:columns>
            <mx:DataGridColumn dataField="label"
                    editable="false" />
            <mx:DataGridColumn dataField="score"
                    editable="true" />
        </mx:columns>
    </mx:DataGrid>

    <mx:DataGrid id="eventsDataGrid"
            dataProvider="{eventsArrColl}"
            itemRenderer="mx.controls.Label"
            verticalScrollPolicy="on"
            rowCount="9"
            width="100%">
        <mx:columns>
            <mx:DataGridColumn dataField="columnIndex" />
            <mx:DataGridColumn dataField="dataField" />
            <mx:DataGridColumn dataField="itemRenderer" />
            <mx:DataGridColumn dataField="reason" />
            <mx:DataGridColumn dataField="rowIndex" />
            <mx:DataGridColumn dataField="type" />
        </mx:columns>
    </mx:DataGrid>

</mx:Application>

View source is enabled in the following example.