Letterbomb is the latest Wii exploit and is currently the only to way to hack a Wii on System Menu 4.3 without using a game.  It's likely that it will not be ported to any lower System Menus since Bannerbomb is available for those.

Letterbomb uses a yet-to-be-specified exploit in the Wii Messageboard.

 

 

System Menu 4.3 (E, U, K, J)

Required:
* An SD card (Best results are with non-SDHC cards, SDHC will only work on 4.0 or above) or USB device formatted to FAT(32).

* Your Wii's WiFi MAC Address (instructions in obtaining to follow...)

Links:
http://please.hackmii.com/

Wii ----------------------

(If you already know how to get your Wii's WiFi MAC Address, skip to step 2)

1.  Go to your Wii's Settings menu.  Select "Internet" on page 2.  Select "Console Information".  Record your Wii's MAC Address for use in the next step.

 

Computer ----------------------

 

2.  Follow the link provided above to the HackMii download page.

 

3.  Make sure "Bundle the HackMii Installer for Me" is checked.  Select your System Menu region, enter in your Wii's WiFi MAC Address, fill in the captcha, and "cut either wire".

 

4.  Extract the contents of the freshly downloaded .zip file onto the root of your SD card.

 

Wii ----------------------

 

5.  Insert the SD card into your Wii.

 

6.  Go to your Messageboard (button on bottom-right of Menu) and navigate to "Yesterday" (depending on your timezone, the exploit could show up under "Today").

 

7.  Click on the envelope with the pink "Bob-omb" to run the HackMii installer.