I'm having the same problem. Starting a couple days ago, any new profile I made, when I tried to play online multiplayer, it said could not connect to server.
So I went to the Wii menu and deleted the CoD MW3 data, thinking it got corrupted somehow and this might fix it.
Apparently that was the wrong move to make, because now all 4 of my profiles get the same error.
Anybody have ideas on how to fix this? As it is, the game is totally broken for me. I can't play multiplayer at all.
For a while there, hackers were running rampant with account hopping.. also it is possible for them to acquire your who's mac address and ban your mac address from the servers.. granted most of the time this is not permant, but since Activision and Treyarch don't care you might be pretty much screwed.. you can try using another Wii and see if that works, if it doesn't then I'm stumped..