I remeber some Vietnam Veterans (friends of my dad) talking about this topic. Apparently, people are held onto the floor of the helicopter by centrifical force when the helicopter is banking. People are held into place during level flight by gravity. One of the veterans stated "You'd almost have try in order to fall out of a helicopter."
My own experience is on CH-53E's, which has enclosed cabins. I know that aircrew have fallen out of the "Hell Hole" (a trap door in the center of the cabin used for lifting external loads). However, I don't know of anybody else falling from the 53. I'd imagine it is possible for people who aren't strapped in to fall out at the ramp if the A/C is at a severe nose-up attitude. However, I think a person would "Almost have to try in order to fall out of the helicopter."
I know a story about a special ops guy falling off of the ramp of a Chinook in Afghanistan. He was firing his SAW from the ramp when the helicopter had to do some violent manuvering to avoid ground fire. The ramp of a cargo helicopter is a dangerous place for people who aren't strapped in, especially if they aren't part of the helicopter crew. The guy probably didn't have time to get off of the ramp or strap himself in, he was too busy returning fire. He probably saved that Chinook and everybody on it.
Semper Fi,
Chris