Almost as ubiquitous as removing one's shoes, taking a laptop out of its case and running it through an X-ray in its own plastic bin has become the norm for people passing through airport security.
The practice has its roots in the 1988 bombing of Pan Am Flight 103 over Lockerbie, Scotland. No, laptops weren't around in 1988, but in that case, a bomb was concealed in a cassette player.
Airport security screeners insist that laptops be removed from cases because the complex components inside could obscure a prohibited object inside a passenger's luggage. Recent portable electronics – iPods, cell phones, and even small laptops – are small enough to be exempt from the policy.