Selects the shape nodes for all selected transform nodes. All transform nodes are then unselected. Any additional selected shape nodes remain selected.

Tip! To obtain the list of selected shape nodes -- including shape nodes associated with selected transforms -- in one quick step, use the following MEL command:

ls -sl -dag -lf;

To replace this script with a one-line MEL command, use the following:

select -r `ls -sl -dag -lf`;

Thanks to Julian Mann for alerting me to this technique!