Digital cell phones more commonly use the 1900MHz band (1.9GHz not far from the Wi-Fi at 2.4GHz). These phones have a transmit power up to .6 Watts, greater than the typical Wi-Fi radio (.02-.1 Watts). More importantly, a cell phone is held right next to the head. Power drops as 1/radius^2 from the source, and the fields near the radiator will be stronger in the higher dielectric medium of the head. A valid study would be with lower power transmitters and reasonable distances (from a few feet and out) with long exposure times.

Cell phone base stations BTW crank out *much* more power than a Wi-Fi radio is allowed to by the FCC. Though this may change down the road as high-power applications (WiMax) are allowed for licensed spectrum holders.