The Wii cannot do 1080p trough a firmware update. It does not have enough GFX memory to make a framebuffer at that resolution. Not that this matters, its video DAC does not support resolutions above 576p anyway so it couldn't project such images even if it did have the memory.
And even if it did magically get better hardware through a software update, it would run into framerate, texture quality and processing power limits. (Just like the HD consoles, but much worse).
Really, HD is not needed for the Wii to be a succes. Wishing for it to be retrofitted on there is silly, the point of the device is to not get caught up in the hardware race.