The recent activity on this thread raised my interest so I checked out the
original Bloomberg article that MacRumours quoted. Apparently the 14 year old was assisted with the study by her father who is a cardiac electrophysiologist. They said 30% of the 26 patients tested had their defibrillator triggered to "Magnet mode" by the iPad covers.
An Apple officer declined to comment and directed attention to the iPad safety instructions which cautions users about radio frequency interference, suggests that patients with pacemakers keep the iPad at least six inches away and says they should be turned off in health-care facilities when instructed by staff or posted signs.
Maybe the sign should read
"All care taken but no responsibility accepted"
EDIT: Without searching the safety instructions the quote above seems to refer to RF not magnets.