The reason the screen does not dim is because dimming of the screen will
disable your AppleTalk connection, so the decision was made for Macintosh
Portable computers connected to a LocalTalk network that the screen will
not dim even if the "sleep" timers expire.
If you are not connected to a LocalTalk network, make AppleTalk inactive,
and the screen will dim as you would normally expect. Of course, based on
the above information, if you are connected to a LocalTalk network you do
not want the screen to dim.