When our eldest was a baby we tried the advice in some parenting books to let her "cry it out" and learn to just go to sleep without being held. I was able to stand her crying for about 15 minutes, uttered an expletive that became famous in the family, and picked her up. End of experiment.
Now Penelope Leach, the favorite parenting advice source of the knowledge class, argues that crying it out is so stressful to babies that they can be scarred, even brain-damaged, by the high cortisol levels this experience induces.
I will be interested in how the scientific argument develops. As for me and my house, we will hold the baby to sleep. More fun for us, too, in the long run.