This is part of the current wonderful inconsistency in Ubuntu which Mark S is gradually working through.
You may not like his approach, but by the end it will be consistent...
Meanwhile, to add or remove the "universal access" icon from the notification area, either Alt+F2 to run:
gnome-keyboard-propertiesOr through the menu's: System > Preferences > Keyboard
Under the Accessibility tab, un/check the box that says "Accessibility features can be toggled with keyboard"
The Accessibility icon should appear/disappear right away. RC