The best way to avoid notice is not to be seen. To that end, and of particular interest to those in non-permissive environments, T3 Tactical presents their Invisible Plate Carrier. T3, which is based in Coronado, is perhaps best known for their Geronimo 2 plate carrier and outside the wire surfboard deployment.
At just shy of ten grand it's a little pricey, but this sort of technology doesn't come cheap. It's very nature presents a few drawbacks, for instance as one user put it, "If you thought MOLLE was a bitch to weave, just wait 'til you try weaving invisible MOLLE."
Buy yours here: http://www.t3gear.com/product_details.php?id=4622