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
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...)
Letterbomb Exploit: http://please.hackmii.com/
0. Make sure the date and time is correctly configured on your console.
(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.
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.
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" or another close-by date).
7. Click on the pink envelope with the bomb to run the HackMii installer.
8. You will see a warning related to scammers (never pay for this software)
Wait for the message at the bottom to appear, then press 1.
9. You will see a screen like this one:
Depending on your wii, it will show different things behind BootMii:.
If you see Can be installed, you can get BootMii as boot2 (which gives the best brick protection there is).
If you see Can only be installed as an IOS, you can only get BootMii as IOS (which will give you NO brick protection on its own).
Press A to continue.
Now we get to the main menu, where we can install everything.
First install the Homebrew Channel (choose Yes, continue).
The Homebrew Channel will now be installed on your Wii.
Optional (Recommended) Steps: Installing BootMii
BootMii is a helps greatly with Brick protection and it is highly recommended to install it.
In the HackMii main menu, choose BootMii... and press A. You will get another menu.
10. Install BootMii as IOS. This will always work.
11. If you're one of the lucky winners, choose install BootMii as boot2, then Yes, continue and let it install.
Once you're done, return to the Main menu and choose Exit.
It will automatically launch the Homebrew Channel. Unless you already downloaded some apps, you will probably see just the background.
You can press Home to bring up the menu and reboot the Wii.
If you installed BootMii
If you installed BootMii, regardless whether it was as boot2 or IOS, it is recommended you use it to create a NAND backup. This backup can be used to restore the Wii to a working state in case you brick it.
You can learn about launching BootMii and making a backup on the BootMii page.
If you installed BootMii as Boot2, the BootMii menu will appear every time you boot your Wii.
If you don't want this, rename/move the "bootmii" folder on the SD card.
Alternatively, you can enable Wii menu auto-boot in the configuration file.
So, what's next?
It mostly depends on what you want to do.
I want to run basic homebrew applications:
I want to load backup games from a USB drive, install WADs:
=> Continue on to install a cIOS which will allow you to use these more advanced apps.
I want to change the system menu theme: