Is it possible to completely disable the keyboard backlight on a MacBook Pro?

Go to System Preferences -> Keyboard and, in the Keyboard tab, turn off (uncheck) "Automatically illuminate keyboard in low light". Then, if you turn the brightness down all the way, it should never turn on. If you then want it on (say, a friend who is not a touch-typist), just use the brightness keys on the top row.

New for 2014: You'd think you should be able to take care of this in one location but, no.

1. Open System Preferences, then open Keyboard, and in the first 'Keyboard' tab panel, uncheck "Adjust keyboard brightness in low light". Also here, be sure to check the "Use all F1, F2, etc. keys as standard function keys" checkbox so that your 'fn' key works. Close the panel.

2. While holding down the fn key, press F5. You should see a semi-transparent control with a setting sun icon. Tap or hold until the bars go down to 0.