Use of Cellular Telephones in the Hospital Environment is a report from the Mayo Clinic. The conclusion:
Interference of any type occurred in 0 of the 75 patient care rooms during the 300 tests performed. These 300 tests involved a total of 192 medical devices. The incidence of clinically important interference was 0% (95% confidence interval, 0%-4.8%).
Of course there are non-medical reasons — annoyance, for example — to not permit cell phones in hospitals. Personally I enjoy the “desert island” feel that comes from being forced to turn off my phone when entering the QEH.