recently released Wii Menu Update 4.2 has killed their console. The patch was designed to remove homebrew applications from modded consoles, providing users with the following warning: "Because unauthorized modifications to save data or program files may impair game play or the Wii console, updating to Wii Menu version 4.2 will also check for and automatically remove such save data or program files." However, a number of Nintendo forumites are complaining the update has bricked their completely unmodified Wiis.
This problem can be attributed to the fact that the update makes changes to the Wii's all-important bootloader. If any problems arise during installation, the bootloader becomes corrupt, and the Wii becomes incapable of loading any software. A Nintendo technician has responded to the reports, saying anyone who's experienced these problems on their totally legit hardware can send it in for free repairs.
In the mean time, you should probably shy away from installing Update 4.2. Unless, of course, you're using a modded console. In that case, you totally deserve your comeuppance.