Thanks to this hint (and its original version, appearing as a link to the author's blog in this comment) about using gdb to find hidden preferences, I have discovered that it's still possible to enable automatic mouseover highlighting in stacks as you could in 10.5 (original hint).
It turns out that the name of the preference key has changed from mouse-over-hilte-stack to mouse-over-hilite-stack. So to enable mouseover highlighting of Stacks, open Terminal and run these two commands:
$ defaults write com.apple.dock mouse-over-hilite-stack -boolean yes
$ killall Dock
After that, your Dock's stack items will highlight as soon as you move the mouse over them.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20090917052821357