According to the FAA, a laser pointed at an aircraft can temporarily blind a pilot and threatens the lives of the passengers and flight crew on board the plane. They have documented over 3,000 incidents of lasers being pointed at airplanes. The effect of lasers pointed at an airplane can be especially dangerous at night when pilot's eyes are accustomed to the darkened cockpit. Fortunately, so far there have not been any airline crashes attributed to the malicious use of lasers.
Lasers have many legitimate uses including laser light shows, astronomical research and satellite imaging. In 1995 the FAA set standards for the use of lasers in proximity to potential flight zones. While this reduces the risk of legitimate laser use interfering with planes, it does not preclude interference from malicious use or from those who may be ignorant of the dangers.
As lasers sold to individuals have become increasingly powerful the risk from malicious or ignorant use has increased. In the last five years the FAA his instituted a reporting process to help identify and mitigate this danger.
Source: King5.com Laser flashes returning Seahawks charter plane January 17, 2011, FAA Laser Hazards In Navigable AirspaceLaser Hazards In Navigable Airspace