For the most part, it's pretty difficult to get a virus on a PC browser, unless you are a complete tool. I don't think Wii virii will be on the horizon.
viruses affect 1) OS 2) specific program (dialer) 3) third party software as relate to #2...
All the above relate to OS, which is Wii made. since opera for wii is pretty much limited functions (aka cant download and install programs), virus can't affect it much.