Backscatter technology exposes travelers to a small amount of radiation as they walk through the scanner; a concern especially for pilots, frequent fliers, and children, who, some say, could have to worry about skin cancer. According to the director of the Center for Radiological Research at Columbia University, the risks associated with one-time exposure are very, very low. But every time a person is exposed to radiation, the small associated risk is multiplied by the number of exposures. If you're a person who flies several times a month (or each week), the seemingly small risk could potentially turn into a larger problem.
For now (and to protect your holiday sanity), casual travelers — especially those who will be flying for the holidays — shouldn't worry about the effects of these machines.