In 10.3.3 's mail program, with my Logitech two-button, scroll trackball mouse, if I select a closed thread and click both mouse buttons and drag down, the thread will expand.
[robg adds: I can't duplicate this with a Microsoft scrollwheel mouse, so perhaps it's something specific to the Logitech button programming...]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20040413062051414