There are excellent comments under the previous post. I am taking a slightly different direction and so am opening a new slot.
What Lewis did not say, but Jesus might, is that not answering physical violence with physical violence is only a special case of a more general rule of not returning evil for evil. Another of his examples was of being made to carry a burden - of being oppressed. That more general command is repeated by both Peter and Paul.
We gravitate to this particular special case because it is dramatic, and comes up in situations of obvious crisis. It is measurable and obvious. Yet there are many types of attack and many types of revenge. Texan99 has trouble discerning how much of any pacifism might come from physical cowardice, but if we look at other types of evils put on us, such as impoverishment, injustice, humiliation, or insult, we might be better able to see how revenge-prone we are. A person who is "too Christian" to fight you but nonetheless slanders you out of your job over the next year has not in any way turned the other cheek.
In my experience, physical courage is not the same as moral courage, but they overlap. While I have known moral cowards who like to get in fights, and physically timid people who nerve themselves up to endure for conscience sake, I find it more common to find the two in similar quantity. Additionally, the courage to face death and the courage to face pain are not quite the same.