Keyboard shortcut to change the behavior of function keys?
You can use a private.xml like this with KeyRemap4MacBook:

&lt;?xml version="1.0"?&gt;
&lt;root&gt;
  &lt;item&gt;
    &lt;name&gt;test&lt;/name&gt;
    &lt;identifier&gt;test&lt;/identifier&gt;
    &lt;autogen&gt;__DoublePressModifier__ KeyCode::FN, KeyCode::FN, KeyCode::VK_CONFIG_TOGGLE_flipfkeys&lt;/autogen&gt;
  &lt;/item&gt;
  &lt;item&gt;
    &lt;name&gt;flipfkeys&lt;/name&gt;
    &lt;identifier vk_config="true"&gt;flipfkeys&lt;/identifier&gt;
    &lt;autogen&gt;__KeyToKey__ KeyCode::BRIGHTNESS_DOWN, KeyCode::F1&lt;/autogen&gt;
    &lt;autogen&gt;__KeyToKey__ KeyCode::BRIGHTNESS_UP, KeyCode::F2&lt;/autogen&gt;
    &lt;autogen&gt;__KeyToKey__ KeyCode::EXPOSE_ALL, KeyCode::F3&lt;/autogen&gt;
    &lt;autogen&gt;__KeyToKey__ KeyCode::LAUNCHPAD, KeyCode::F4&lt;/autogen&gt;
    &lt;autogen&gt;__ConsumerToKey__ ConsumerKeyCode::KEYBOARDLIGHT_LOW, KeyCode::F5&lt;/autogen&gt;
    &lt;autogen&gt;__ConsumerToKey__ ConsumerKeyCode::KEYBOARDLIGHT_HIGH, KeyCode::F6&lt;/autogen&gt;
    &lt;autogen&gt;__ConsumerToKey__ ConsumerKeyCode::MUSIC_PREV, KeyCode::F7&lt;/autogen&gt;
    &lt;autogen&gt;__ConsumerToKey__ ConsumerKeyCode::MUSIC_PLAY, KeyCode::F8&lt;/autogen&gt;
    &lt;autogen&gt;__ConsumerToKey__ ConsumerKeyCode::MUSIC_NEXT, KeyCode::F9&lt;/autogen&gt;
    &lt;autogen&gt;__ConsumerToKey__ ConsumerKeyCode::VOLUME_MUTE, KeyCode::F10&lt;/autogen&gt;
    &lt;autogen&gt;__ConsumerToKey__ ConsumerKeyCode::VOLUME_DOWN, KeyCode::F11&lt;/autogen&gt;
    &lt;autogen&gt;__ConsumerToKey__ ConsumerKeyCode::VOLUME_UP, KeyCode::F12&lt;/autogen&gt;
    &lt;autogen&gt;__KeyToKey__ KeyCode::F1, KeyCode::BRIGHTNESS_DOWN&lt;/autogen&gt;
    &lt;autogen&gt;__KeyToKey__ KeyCode::F2, KeyCode::BRIGHTNESS_UP&lt;/autogen&gt;
    &lt;autogen&gt;__KeyToKey__ KeyCode::F3, KeyCode::EXPOSE_ALL&lt;/autogen&gt;
    &lt;autogen&gt;__KeyToKey__ KeyCode::F4, KeyCode::LAUNCHPAD&lt;/autogen&gt;
    &lt;autogen&gt;__KeyToConsumer__ KeyCode::F5, ConsumerKeyCode::KEYBOARDLIGHT_LOW&lt;/autogen&gt;
    &lt;autogen&gt;__KeyToConsumer__ KeyCode::F6, ConsumerKeyCode::KEYBOARDLIGHT_HIGH&lt;/autogen&gt;
    &lt;autogen&gt;__KeyToConsumer__ KeyCode::F7, ConsumerKeyCode::MUSIC_PREV&lt;/autogen&gt;
    &lt;autogen&gt;__KeyToConsumer__ KeyCode::F8, ConsumerKeyCode::MUSIC_PLAY&lt;/autogen&gt;
    &lt;autogen&gt;__KeyToConsumer__ KeyCode::F9, ConsumerKeyCode::MUSIC_NEXT&lt;/autogen&gt;
    &lt;autogen&gt;__KeyToConsumer__ KeyCode::F10, ConsumerKeyCode::VOLUME_MUTE&lt;/autogen&gt;
    &lt;autogen&gt;__KeyToConsumer__ KeyCode::F11, ConsumerKeyCode::VOLUME_DOWN&lt;/autogen&gt;
    &lt;autogen&gt;__KeyToConsumer__ KeyCode::F12, ConsumerKeyCode::VOLUME_UP&lt;/autogen&gt;
  &lt;/item&gt;
&lt;/root&gt;


It makes pressing fn twice toggle between using the F-keys as normal function keys.