We have a wireless sender unit which we use to send the Sky signal from the main box in the lounge to a satellite TV in the kitchen (easy option, cabling would have been a nightmare). Quite often if the sender is on it bugga's up the signal from my Wireless broadband router and her indoors can't capture the broadband signal on her laptop: solution - switch TV signal sender off, changing the channels on it doesn't work.
A friend had the same problem a few months ago and found he couldn't set up his home wireless network he was well puzzled so called in an 'expert' who couldn't figure it out either so proceeded to hard wire all my friends PC's (his primary desktop, his secondary (guests) desktop and his kids laptops x 2!) back to the router at great expense! wasn't until I popped over to visit and spied the TV signal sender sitting in the corner of his lounge that the real culprit was uncovered. Sure enough we switched off his TV signal sender and Hey Presto! he could capture the broadband signal anywhere in the house on his kids laptop.
Don't have a Wii so not sure how they 'send' but perhaps they cause the same 'signal jamming' problem as the TV signal sender? it could be the signal between the Wii and the games controllers (wands, bats, whatever)that are causing you the grief?
