I always opt out. I do not believe the US government should require nude pictures of all pax before domestic travel, as it is beyond the purview of "administrative search."

The MMW *may* be safe in that it is not proven carcinogenic yet, but the lag time from exposure to development of a cancer is apt to be at least 15 - 20 years, so we really do not know at this point.

More importantly, I think most TSA clerks do not like doing pat downs; therefore the pat down degrades the clerk more than it does me, as I have "gotten used to them." If the government is requiring this foolish security theater, I think it is appropriate that it discomfit the perpetrators in a way that hiding in a peep show box or looking at a gumby figure does not.