From the Salon.com story by Dave Lindorff:
So what was it? Jacob McKenna, a spyware expert and the owner of the Spy Store, a high-tech surveillance shop in Spokane, Wash., looked at the Bush image on his computer monitor. "There's certainly something on his back, and it appears to be electronic," he said. McKenna said that, given its shape, the bulge could be the inductor portion of a two-way push-to-talk system. McKenna noted that such a system makes use of a tiny microchip-based earplug radio that is pushed way down into the ear canal, where it is virtually invisible. He also said a weak signal could be scrambled and be undetected by another broadcaster.
... [Critics] say the president and his handlers may have turned to a technique often used by television reporters on remote stand-ups. A reporter tapes a story and, while on camera, plays it back into an earpiece, repeating lines just after hearing them, managing to sound spontaneous and error free.
Suggestions that Bush may have using this technique stem from a D-day event in France, when a CNN broadcast appeared to pick up -- and broadcast to surprised viewers -- the sound of another voice seemingly reading Bush his lines, after which Bush repeated them. Danny Schechter, who operates the news site MediaChannel.org, and who has been doing some investigating into the wired-Bush rumors himself, said the Bush campaign has been worried of late about others picking up their radio frequencies -- notably during the Republican Convention on the day of Bush's appearance. "They had a frequency specialist stop me and ask about the frequency of my camera," Schechter said. "The Democrats weren't doing that at their convention."