Here is how you would do it with the GridView:
<AjaxData:GridView runat="server" ID="gridview1" RowDataBoundEvent="onRowDataBound">
<Columns>
<AjaxData:GridViewBoundColumn DataField="Id" HeaderText="Id" />
<AjaxData:GridViewBoundColumn DataField="Username" HeaderText="Username" />
</Columns>
</AjaxData:GridView>
function onRowDataBound(sender, e)
{
if (e.get_row().get_rowType() == AjaxDataControls.GridViewRowType.Header)
{
var row = e.get_row();
var item = row.get_dataItem();
var idIndex = gridview1.getColumnIndexByHeaderText('Id');
var tdId = row.get_container().childNodes[idIndex];
tdId.onmouseover = function() { this.style.backgroundColor = 'red' }; // Replace this with the js function for the tooltip
tdId.onmouseout = function() { this.style.backgroundColor = '' };
}
}