But if the update makes the Wii able to read Dual Layer, wouldn't that mean, that people who did the 15% trick, wont be able to play the actual game when its dumped (actually they wont even be able to play the original)?
Well, here's what the theory is: the beginning of the update adds dual layer Smash support. So when you shut off your Wii, it has the dual layer support. Smash only prompts the update if it thinks it needs dual layer support, so people who shut off at 15% have dual layer support. So, in theory, a dual layer game works on 15% method. I don't know anyone who has tested it, most people with the single layer version don't actually have a copy of the game.
Also, once SSBB is released in the states/EU, you can always just copy any save data you wish to keep to an SD card, then format Wii memory and it'll get rid of the dual channels. Or so i've heard.