What you need to know about Wii System Menu 3.4.

Nintendo recently offered an update for the Wii software, which offered several improvements, these are:

Support for USB keyboard in the Mii Channel
Enhanced parental controls
Improved SD card read/write speed

One of the negative side effects of this update is that currently, the Twilight Hack will not work with it. This means that if you update to System Menu 3.4, and then wish to run homebrew on your console, you will be unable to do so.

If however, you have already installed the Homebrew Channel, you should be fine. It will not break currently installed Homebrew applications, it will only prevent new installations of the homebrew channel.

It is likely that a workaround will be found soon, so don't despair if you have already updated.

I would recommend that you avoid updating your system in the mean time, unless any of the new features are hugely important to you, or you are happy with your Homebrew Channel install.