Keyboard shortcut for toggling between Fn and F keys

Depending on what you're trying to do, you may be able to cut out the keyboard shortcuts altogether and completely automate this. KeyRemap4MacBook allows you to set key re-mappings based on what application you're currently using.

I am using a setting that switches F1 through F12 to standard Function keys whenever I'm using RDC (Windows Remote Desktop). Any other time they operate like their icons (brightness, volume, play, etc).

There is now an app for this: https://github.com/Pyroh/Fluor

It lets you specify the fn key behaviour depending on a rules engine, and also switches between one behaviour and the other when you press the fn key.

You can create application specific shortcuts by creating a custom setting in your private.xml file. Check out the reference documentation for details, or my example below where I'm rebinding the Consumer keys back to Function keys when in PhpStorm (to use the debugger).

<?xml version="1.0"?>
<root>
    <!-- PhpStorm & EAP Version -->
    <appdef>
        <appname>PHPSTORM</appname>
        <equal>com.jetbrains.PhpStorm-EAP</equal>
        <equal>com.jetbrains.PhpStorm</equal>
    </appdef>
    <!-- ============================================================ -->
    <item>
        <name>Switch to normal function keys when in PhpStorm</name>
        <appendix>Change consumer keys to function keys in PhpStorm</appendix>
        <identifier>private.app_phpstorm_switch_consumer_to_fn</identifier>
        <only>PHPSTORM</only>
        <autogen>__KeyToKey__ KeyCode::BRIGHTNESS_DOWN, KeyCode::F1</autogen>
        <autogen>__KeyToKey__ KeyCode::BRIGHTNESS_UP, KeyCode::F2</autogen>
        <autogen>__KeyToKey__ KeyCode::EXPOSE_ALL, KeyCode::F3</autogen>
        <autogen>__KeyToKey__ KeyCode::LAUNCHPAD, KeyCode::F4</autogen>
        <autogen>__KeyToKey__ KeyCode::VK_CONSUMERKEY_KEYBOARDLIGHT_LOW, KeyCode::F5</autogen>
        <autogen>__KeyToKey__ KeyCode::VK_CONSUMERKEY_KEYBOARDLIGHT_HIGH, KeyCode::F6</autogen>
        <autogen>__KeyToKey__ ConsumerKeyCode::MUSIC_PREV, KeyCode::F7</autogen>
        <autogen>__KeyToKey__ ConsumerKeyCode::MUSIC_PLAY, KeyCode::F8</autogen>
        <autogen>__KeyToKey__ ConsumerKeyCode::MUSIC_NEXT, KeyCode::F9</autogen>
    <!-- I want to use these while in PhpStorm
        <autogen>__KeyToKey__ ConsumerKeyCode::VOLUME_MUTE, KeyCode::F10</autogen>
        <autogen>__KeyToKey__ ConsumerKeyCode::VOLUME_DOWN, KeyCode::F11</autogen>
        <autogen>__KeyToKey__ ConsumerKeyCode::VOLUME_UP, KeyCode::F12</autogen>
    -->
    </item>
</root>

There might be an easier way, but the following comes to mind:

• Turn on Universal Access.
• Create an AppleScript or Automator App that toggles this via the UI.
• Bind that app to a key combo.