Ghosth,
I did a little checking and found this, in the red text below, on a thread in the Wii support forums. I don't know how knowledgeable this person is but maybe he is wrong or someone has found a "work around" solution.
The wii system software has no functionality to read from a USB pen drive.
Currently the only uses for the USB ports are the LAN adapter, a standard USB keyboard (Which can be used to enter text on the wii message board, any standard Wii channel or system menu, and the Internet Channel) and unauthorized third-party devices like cooling fans and such that you don't really need unless your wii is situated in a small oven.
Sorry,
rkanjl