The TV is indicating the signal it's receiving, which is 576i. You can get the Wii component cable which will change the signal to 480p - should be an improvement as it's a progressive rather than interlaced signal. However, without putting the signal through something else e.g. an AV amp with upscaling abilities, there's no way to change this input signal.
The TV will actually be upscaling it to the TV's native resolution of 1080p anyway, so there's no need to actually get the source upscaled before it gets to the TV. Considering WHF's review of the set, I would think you would have to be spending quite a bit of money on an upscaler to be improving the picture.