The ItemRenderer class in Flex 4 has an itemIndex property that was added after beta2. You can use this property within your item renderer to display the row number or index of an item in a spark List.
The following example demonstrates a List with a custom item renderer that uses this itemIndex property. If you look at the commented out code in the item renderer you will also see a way of getting this information using the getItemIndex() method of the dataProvider.
Note: This sample requires Flex SDK 220.127.116.1193 or higher. You can get the latest SDK builds from opensource.adobe.com.