Vader347's version of mplayerwii with the widescreen bug fixed can be found here. Vader347 claims that the only real change is one line of code (as recommended by pepsiman):
"rmode->viWidth = 678; rmode->viXOrigin = (VI_MAX_WIDTH_PAL - 678)/2;" after "rmode = VIDEO_GetPreferredMode(NULL);"
"rmode->viWidth = 678; rmode->viXOrigin = (VI_MAX_WIDTH_PAL - 678)/2;" after "rmode = VIDEO_GetPreferredMode(NULL);"
Download Here and Give Feedback Via Comments