Read it here:
http://www.joystiq.com/2009/10/01/wii-system-update-4-2-bricking-unmodded-consoles/
With that said, please don't apply the system update to your Wii modded or not. Nintendo made this update to delete any custom firmware and exploits. Instead, it has a good chance of screwing up non-modded Wiis. Proceed with caution.
My Wii is filled with custom firmware, Region Changers, emulators, and even a 1 TB harddrive connected to it containing ALL of my games. Obviously I'm not going to bother to update it.