It's all a matter of statistics and crash dynamics.
Yes, a rear-facing carseat is safe when used properly even in rear-end crashes. This scenario is much the same as a front-facing carseat which provides adequate protection in a frontal crash.
Technically, rear-facing carseats are safer in frontal and frontal offset crashes. This is because they support the baby's entire back, neck and head during the crash while a front-facing carseat provides little to no protection for the head and neck. In such crashes, the head and legs of a child in a front-facing carseat are thrown forward like a ragdoll, and younger infants are less able to survive these forces.
In the same manner, you could argue that a rear-facing seat would not be as good as a front-facing seat for an infant in a car that is rear-ended. While this is true in principle, carseat advocates (including the American Academy of Pediatrics) all recommend that infants remain rear-facing to AT LEAST one year AND 20 pounds, and preferably AS LONG AS POSSIBLE up to the limits of the carseat. Why?
1) Frontal and frontal offset crashes are FAR more frequent that rear-end crashes. In fact, they account for about 72% of all crashes according to crashtest.com, while rear and rear-offset crashes only account for about 4%. The remainder are side impacts, where rear-facing carseats may also provide an increased measure of safety.
2) The energy involved in frontal crashes is usually much higher than in rear-end crashes. This is simply because frontal crashes occur at higher speeds, while rear-enders usually occur with one vehicle stopped and the other at a relatively low speed.
3) Frontal crashes are very sudden. With both vehicles travelling in opposite directions, the cars stop almost instantly. This tends to transfer much more power to the passengers because they have less time to ride-down the energy in the crash. More power transferred to internal organs results in greater injuries. Rear-end crashes, on the other hand, often involve a stopped car, or two cars travelling in the same direction. The car in front is usually bumped forward and does not come to a sudden stop, and that means more ride-down time for the passengers. Rear-end crashes can cause serious injury, but the usual injuries are whiplash injuries to adults who have inadquate head/neck support.
For these reasons, it is best to protect infants against the crashes which are by far the most common and most severe. That is why the universal consensus among carseat experts is that rear-facing is safest. Some countries like Sweden keep their children rear-facing to 3-4 years, and their rates of injury are practically zero for those age groups.
Also see:
www.parentsplace.com/babi...76,00.html
www.aap.org/family/famshop.htm