Holding the iPod just two inches away from their chests for 5 to 10 seconds was enough to cause electrical interference half the time. In some cases, iShenanigans could be detected as far as 18 inches away. The interference usually just caused the equipment to misread the heart’s pacing, but one case caused the pacemaker to stop working entirely.
No other music players were tested, although I’m sure giving grandpa a Zune with some preloaded Slayer would have a similar effect.
Source: The Register