Well... the virus would have to be made specifically to harm the Wii. That really isn't a very large demographic. It's the same reason we see tons of viruses on PC's and not MAC's. It's really because it isn't worth a hackers time too infect the small number of MAC users compared to PC users. And believe it or not, these viruses usually are created for a purpose that is not simply to destroy your system. A lot of the time, viruses simply turn your PC into a zombie which proceeds to send data to a server whenever it is connected to the internet. When there are also 2 million other PC's doing this simultaneously, the server overloads and malfunctions. So what the hackers are generally trying to do is break that server.
On the Wii, though, the virus would need to be able to be downloaded. From what I know, nothing can be downloaded without you first telling it too. Second, I don't think anything can be run on the Wii without you initializing it. And the problem there lies in that it would simply be stored as saved data, without an executable to launch it.
So in a shorter version of what I just said, we are perfectly safe!