Going from sub/superscript to regular baseline with keyboard shortcut in Keynote
Pressing ⌃⌘+ or ⌃⌘- again seems to remove superscript or subscript in Keynote '09.

Other applications like TextEdit raise or lower the text multiple times though.

You could give custom shortcuts to nested menu items on 10.7, but it stopped working on 10.8.



Another option would be to create ~/Library/KeyBindings/ and save a property list like this as DefaultKeyBinding.dict, but it doesn't seem to work in iWork applications.

{
    "^@=" = unscript:;
}


You could also assing a shortcut to a script like this:

try
    tell application "System Events" to tell (process 1 where it is frontmost)
        click menu item "Use Default" of menu 1 of menu item "Baseline" of menu 1 of menu item "Font" of menu "Format" of menu bar 1
    end tell
end try