hi
I am a Radiologist by profession and not exactly an expert on the question which has been asked. However I shall try and answer it to best of my knowledge.
Firstly the immediate problem of working with your lap top on your legs for prolonged duration can be due to postural fatigue, leading to pain in the neck(literally) and thoracic spine.
Now regarding the carcinogenic effect, there have been various hypotheses but no definitive evidence is forth coming. It is a known fact that electro magnetic radiation is indeed emitted from the laptops/computers/TV and so on. But equally known is the fact that total dose of man made radiation and due to cosmic rays is about 02 msv/yr, where as a radiation worker can be exposed to 50 msv in a year. so it may not be of much concern. However I personally feel that if you keep laptop on your laps it is close to gonads and these organs of the body are really sensitive to the radiation, so why take a chance. However if you have completed your family than may be you can go ahead for marathon sessions of laptopping on your laps. As far the original query is concerned I have not come across any compelling evidence in the literature to suggest any predisposition to cancer.
regards