Rory,
The third argument to create a list box is an optional array of strings that it uses to create items.
To make it work you need to have undefined as the third argument (or pass it an array of strings):
d.folderPath = folderPathGroup.add( "listbox", undefined, undefined, {multiselect:true} );
d.folderPath.preferredSize = [400,100];
Bob
Adobe WAS Scripting