The flex datagrid supports an event handler called itemDoubleClick , however one little gotcha is you also need to set doubleClickEnabled to true in order for this event to be properly fired.
<mx :Script>
< ![CDATA[
protected var dp:ArrayCollection = new ArrayCollection();
protected function trackGrid_itemDoubleClickHandler(event:ListEvent):void {
var data:Object = dp.getItemAt(event.currentTarget.selectedIndex);
// TODO: what you want to do with the data here
}
]]>
</mx>
<mx :DataGrid dataProvider="{dp}" doubleClickEnabled="true" itemDoubleClick="myGrid_itemDoubleClickHandler(event)" id="myGrid">
</mx>