I can't seem to figure out how to get my DataTips to render for a single Column of data in my DataGrid.  The following code renders DataTips for every column of data and not just the "lastActivity" dataField as desired.  If I pull the showDataTip="true" prop out of the DataGrid and place it under the DataGridColumn, no DataTips are rendered.  Does anyone know what the trick is to get this to work? 

PS: My dataTipFunction is working fine.

<salesforce:Connection id = "apex" />
    <mx:DataGrid id="bg" x="0" y="66" width="100%" height="50%" editable="false"
            sortableColumns="true" resize="true" doubleClickEnabled="true"
            showDataTips="true" dataTipField="lastActivity" dataTipFunction="dataTipUpdate"
            itemDoubleClick="true" itemClick="itemClickEvent(event);">
           
        <mx:columns>
            <mx:DataGridColumn headerText="Name" dataField="name"/>
            <mx:DataGridColumn headerText="Company" dataField="company"/>
            <mx:DataGridColumn headerText="Download Date" dataField="downloadDate"/>
            <mx:DataGridColumn headerText="Product" dataField="product" sortable="false"/>
            <mx:DataGridColumn showDataTips="true" headerText="Last Activity" dataField="lastActivity"/>
               
        </mx:columns>
    </mx:DataGrid>

Thanks!
-Paul