Posted to Nintendo's support site alongside the update: "Because unauthorized channels or firmware may impair game play or the Wii console, updating to Wii Menu version 4.3 will check for and automatically remove such unauthorized files."
That being said, if you actively use Homebrew channels on your Wii, updating to version 4.3 may not be the best option for you, just yet.
If you do not use any homebrew on the console, the update brings some "behind the scenes enhancements," as well, for improved system performance.
Reads the full update:
Wii Menu 4.3
Unauthorized Modifications
Because unauthorized channels or firmware may impair game play or the Wii console, updating to Wii Menu version 4.3 will check for and automatically remove such unauthorized files. In addition, there are some behind the scenes enhancements that do not affect any prominently-used features or menus but will improve system performance.
Written by: Andre Yoskowitz @ 23 Jun 2010 21:20