From Nintendo's Wii Message regarding the update, sent just a few hours ago to worldwide Wii units, 4.2 does some improvements behind the scenes. For legit console users, this seems to mean that older game disks are running a little better, though this has yet to be verified.
But for those of us using the popular installed Homebrew Channel, it's a very different tale. Firmware Update 4.2 completely REMOVES the Channel, and all known installations regarding it, including the region-free player, the USB Loader, and even the DVD Player.
Needless to say, the homebrew community will eventually find a way around this, but this is certainly the biggest attack on potential Piracy yet by Nintendo.
It seems that the update is also adversely affecting Wii owners without Homebrew as well. Many users have reported their consoles completely bricking during the installation process of 4.2. Technically it is because if 'boot2', one of the wii's bootloaders, encounters any errors during its update, like a loss of internet connection for example, it completely freezes, and doesn't allow the menu to appear upon switch-on.
Of course, this is of no consequence to those of us that have updated already, but to everyone else, be careful. Nintendo should address this soon, or at least have a truckload of consoles to repair.