According to a recent BBCNews report titled 'Laptops may damage male fertility':
Balancing it on the lap increases the temperature of the scrotum which is known to have a negative effect on sperm production, researchers found.
Coupled with the rising popularity of these computers - about 150m people use them worldwide - much more research is needed say the US authors.
The State University of New York findings appear in Human Reproduction.
The moral is - be careful where you balance!