There has been some suggestion that men using a laptop on their knee may have a negative impact on their sperm count... not a good thing if you are planning a family.
Although the evidence is quite weak it may ultimately prove to have some validity. The problem revolves around the heat generated by the laptops which in turn heats up the testicles which in turn could impact on the quality of sperm.
Probably worth keeping it in mind if you spend a lot of time with a laptop on your knee. There is a Reuters article discussing this which you can read by clicking here ("Is your laptop cooking your testicles?" by Reuters) .