Wireless gamepads are more convenient, but for latency, copper trumps 2.4GHz. What to do? Build hybrid controllers that can instantly cut the cord. It seems that's what Microsoft's thinking, as the company applied for a patent on just such a controller in March, using infrared, Bluetooth or RF technologies alongside a wired (possibly USB) connection and switch between the two "without permanent disruption to an ongoing game." Of course, dual-mode controllers have been around for months in a slightly different form -- Microsoft's own Sidewinder X8 and the Razer Mamba did the same for the high-end gaming mouse. It also doesn't bode well that this particular application clearly describes the original chunky Xbox.
Note: As some of you have surmised in comments, this is similar to how the PlayStation 3's controllers work, though they hardly perform the task seamlessly. Meanwhile, Microsoft's Xbox 360 Play & Charge kit doesn't interrupt a game, but only uses the physical cable to charge a controller's battery pack.