Privacy and Safety

Traveling day, so it seems an appropriate time to ask about privacy and safety in the air. In light of the BVD bomber's attempt, full body scans are being employed. These are made by imaging machines that let security folks look at you without your clothes. Clearly, the next step is mandatory cavity searches (if they were smart, Homeland Security could be folded into the health care bill, so that the orifice checks could be done by urologists thereby fighting terrorism and prostate disease at the same time).

The question is where the privacy line is. Is there any point at which we can say that the risk involved is outweighed by our personal right to bodily autonomy or privacy? If so, where is that line and why?