EMI with enough power could certainly cause problems with instrumentation. But a cell phone back in the passenger cabin doesn't come close to having enough power. To reiterate from another thread, Boeing, Airbus, the FAA and others did tests with many different cell phones onboard many different aircraft and found zero problem with interference either with the cockpit or the wiring running through the airplane.

This is why we will soon (2006 probably) be allowed to have our cell phones on the plane. The real problem with cell phones on airplanes is they cause havoc with the cell stations on the ground when moving at 500mph. There is an actual FCC (not FAA) regulation against using cell phones on airplanes. But the new system that AA tried out relays the call to a satellite, thus avoiding problems on the ground.

As for Wifi, most access points start out at a maximum of 50 milli-watts. You can add high-gain antennas, but that wouldn't be necessary on an airplane. So WiFi is absolutely no risk to airplane instruments.