I did some looking into the TSA body scanners. Both times I've come back from the US, I've gone through the scanners only to have them raise some concern about my chest and pat it down. The first time I was asked if I had a big necklace on. This time, he just said he needed to pat down my chest. I tried to figure it out - did my bra set something off? If so, couldn't they see from my outline that I had no penis?
Turns out those scanners that show genitals are long gone as of early 2013. All they see now is a cartoon character, with a yellow mark wherever something suspicious is. They also don't always work that well (even the old ones that showed genitals didn't) so if someone wears baggy clothes or has an unusual form, they pat you down. So one of two things happened:
1) The scanner did in fact detect my bra as an abnormality, and it showed up on the screen.
2) I passed the body scanner, but they were concerned about my baggy sweater combined with an unusually large lump.
So happy: I obviously pass as male, despite my chest. I'm directed to the male officer without question.
Unhappy: my chest looks big enough that they suspect I am smuggling drugs or weapons in it. LOL. I guess it's kind of funny, in a horrible way.
As a sidenote, the male officers are always extremely professional in the chest pat-down. Just take both hands, sideways, and press down on the boobs with their palms. It's very quick and non-invasive.