I have an Apple Airport Extreme WiFi router and three wireless clients (iPhone, MacBook Pro & iMac).
The iPhone and a MacBook Pro are connected to the WiFi network and everything works fine. As soon as I join the WiFi with the iMac, all three lose Internet connectivity and the ping times for external sites go to ~100,000ms at which point the network becomes unusable.
As soon as I shut the WiFi on the iMac off, the network returns to normal. What's interesting is that my WiFi base station has two SSIDs, one private network one guest network. It doesn't matter which network I join with the iMac, both cause the same problem.