A pilot told me a story several years ago of an alleged incident aboard an El Al 747 crossing the Atlantic eastbound. The autopilot kept hiccuping, for want of a better word -- little jolts and engine surges. The first officer went downstairs to have a look out the windows, and discovered a passenger taking pictures with a digital camera. Whenever the shutter button was pressed: hiccup.

So I don't think it's unreasonable to say stray RF or other wireless signals can affect aircraft guidance systems.