Remap CapsLock(tap) to ESC and CapsLock(hold) to CTRL in OS X Lion

Step by step instructions:

1. Install both PCKeyboardHack and KeyRemap4MacBook. It will prompt you to reboot after installing the apps.
2. Disable Caps Lock. Go to System Preferences -> Keyboard -> Modifier Keys. Set Caps Lock to "No Action".
3. In PCKeyboardHack set Cap Lock to Control_L by changing the Caps Lock keycode to 59.
4. In KeyRemap4MacBook, toggle "Control_L to Control_L (+ when you type Control_L only, send Escape". I searched for "Control_L escape" and it showed up near the top.

UPDATE:

On High Sierra, this functionality is built into the new Karabiner Elements.

Follow the install instructions, then:

1. Open Karabiner-Elements
2. Click "Complex Modifications"
3. Click "Add Rule"
4. Click "Import more rules from the internet"
5. On the resultant web page, look for "Modifier Keys > Change caps_lock Key" and import the configuration.
6. Profit!

This worked for me on OSX 10.9:

1. Set Caps Lock to Control. Go to System Preferences -> Keyboard -> Modifier Keys. Set Caps Lock to "^ Control",
2. Install KeyRemap4MacBook,
3. In KeyRemap4MacBook, toggle "Control_L to Control_L (+ when you type Control_L only, send Escape".