Of course, they could. Right now, the wii domain names are helping their hype. Fortunately, three letter combinations like "wii" have been used long before this game system. The fact that Nintendo doesn't even own wii.com shows that the letter combination has had previous and popular use.
Nevertheless, Nintendo has a lot more money than all of us, and most of our domains probably wouldn't be worth defending (ie. hiring a lawyer, etc).
Wahoo! Wiigii!
Jag