I do not recall if this was present in 10.5, but at any rate, it's a feature of Snow Leopard -- just hold down Command prior to clicking any item in a Stack, and that item's folder will open in the Finder.
[robg adds: I believe this worked in 10.5, too, and I thought for sure we'd documented it here before. However, my searches came up blank, so here it is.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20090920112913490