Hi Joe,
Here are some OSHA references for portable ladders.
1926.1053(b)(13)
The top or top step of a stepladder shall not be used as a step.
1926.1053(b)(4)
Ladders shall be used only for the purpose for which they were designed.
When reading the manufacturer's instructions attached to a portable ladder, you will see that it says something about "not standing above this step" which is the step before the "top step".
1926.1060(a)
The employer shall provide a training program for each employee using ladders and stairways, as necessary. The program shall enable each employee to recognize hazards related to ladders and stairways, and shall train each employee in the procedures to be followed to minimize these hazards.
Hope this helps!
Dave
[This message has been edited by WebSparky (edited 02-25-2003).]