I don't know if I've just been in the dark about this, but I just discovered a freebie called FMMenulet, which places an icon in the menubar to alert you as to whether Script Debugging is on or off (red or green icon), and to toggle debugging by simply clicking the icon.

It is said to work in Developer/Advanced from 6 on up. Sorry, Mac 10.4 only.

See http://www.fmdiff.com/fmmenulet.html

Matt