They've done it. The hackers of the Wii have finally cracked open System Menu 4.0! What you do is:
Rename the folder named "private" on your SD Card to "privateold" (without the quotes)
Put the folder "private" (the new one) on the root of your card.
Remove the the disk from your Wii, if one is inserted.
On your Wii go to: Wii Options -> Data Management -> Channels -> SD Card
Insert your SD Card
A menu should pop up asking if you want to run the boot.dol/boot.elf file on your SD card. Say yes, and it'll run whatever file you have on your card root named boot.dol or boot.elf. I'd suggest running the HackMii installer, so that you can install the Homebrew Channel, DVDx, and BootMii all at once. Please go to Wiibrew.org, or hbc.hackmii.com for information on the mentioned pieces of software. Good luck!