I'm no lawyer, but thinking like one...
If homebrew is indeed legal, this update may remove homebrew, but I think if it bricks your wii, Nintendo could be held responsible for some sort of malicious intent or perhaps destruction of property... I could be wrong about that. I don't think Nintendo would pull a jerk move like that anyway even if they couldn't be sued (They're trying to stop piracy, not punish it--I think), so I doubt that brickage is something to worry about.
EDIT: I thought of this based on EA Games getting sued--but now that I think about it, that was a PC game, and people have much more rights when it comes to computers. So instead of might be wrong, let's say there's a good chance that I'm wrong.
Edit again: My point was that people shouldn't worry about their wii getting bricked. But first of all, apparently Nintendo has the right to brick. And second of all, it is apparently something to worry about. So nevermind.