I had a good couple of months dificulty getting my Wii to connect, I google searched, tried changing channels, closely examined all bulitin boards for issues, but finally though of an idea myself and found that dispite all my other computers being able to connect to my WIFI network, a small radio frequency (RF) TV signal sender, I was using was interfereing with the Wii, I then changed the frequency of this sender and the Wii connected imediately, with only one brief down time since, which i have no idea why this one occured.
So do see if any other RF interference could be at work, which i suppose could include microwaves, mobile Phones, DECT phones, CBs etc. and good luck with it, it certainly annoyed the hell out of me