Similar to what's admitted in the iPad's user manual, the World Health Organization's "Base stations and wireless technologies Fact sheet" states that exposure near the "source" of the radiation "may exceed international exposure limits."
Radiation of iPad with WiFi ON but no Internet Connection http://youtu.be/ANH6PEyQSZY
* As shown in the video at the bottom of the above page, the measurement of magnetic fields at the back of the iPad (free of WiFi's RF signals) shows that even when the iPad is not on wireless mode, it's not advisable to place the unit on the lap.
Updated Medical Advisory on WiFi Radiation and Children's Health from governmental bodies and doctors' associations:
21st Century Learning: Higher Radiation from iPads on WiFi than Cellphones that are Actively Transmitting