IANAL, but doesn't prior-art have to be published to count?
The purpose of this is to establish that Samsung did not wilfully infringe even if Apple's patents are valid (they will argue they were just in good faith continuing to evolve designs they already had that were similar), but that Apple's patents are invalid because they are dictated by function - it is logical for touch screen phones to get a large screen and reduce number of buttons, for example - and because the design elements were obvious (arguing that the existence of a wide range of similar designs, even if not published, shows that no knowledge of the iPhone was necessary for someone with ordinary knowledge of the arts to come up with something similar).
Samsung has previously argued prior art previously in the form of prototype / design studies for tablets by Knight-Ridder.
As I've said before (http://news.ycombinator.com/item?id=4244317).
Apple can't demonstrate 10, let alone any meaningful number of people who accidentally bought a Galaxy when they meant to pick up an iPad/iPhone.
It doesn't happen.
So yes. It does happen.
"One of these phones (the bottom-right one) became the Samsung F700 - a product Apple once included as an infringing product, but later withdrew once it learned Samsung created it and brought it to market before the iPhone"
Secret prototypes would not be prior art, though.
Samsung might be well within their rights if they say "See, we invented it first, but didn't think it was patent worthy." I don't know the particulars on the "patent worthy" consideration.
No, you would need to prove that your documented development of the invention predates the other party's documented development of the invention. Filing date does not matter in that case.