I have a non-Unibody MacBook Pro running 10.8.2. After I installed Mountain Lion, the keyboard backlight does not light up any more. Is it a connection with the new OS, or is it a defect of my Mac?
You're not alone, expect an OS X update to address this very soon, until then others have had success by disabling the auto dim in low light feature.
System Preferences > Keyboard > uncheck "Adjust keyboard brightness in low light".