I've created a custom TreeItemRenderer that does renderer tree lines. It has these properties:
<ui:TreeItemLinesRenderer Styles lineAlpha="1" lineColor="#808080" lineThickness="1" lineStyle="dotted"/>And here is an example of it in action (right click to view source):
If you know of any other solutions out there I'd be interested to see them.
** September 28th 2009 Updated **
I've added another example application showing how to render tree lines in an AdvancedDataGrid control.
Flex 4/Spark Version
Here is a very similar version written in Flex 4. It uses a lot of the same code, but uses a Spark item renderer instead. There is probably a much nicer way to do this using the new Spark Path or Line controls, but since the code was already written I went with this solution.
An example of tree lines in an AdvancedDataGrid with a Flex 4 itemRenderer is on my newer blog post.