Some feminized seeds are created by stressing a female plant to produce pollen sacs, and then using this pollen for fertilization. This is where the idea that feminized seeds are all hermaphroditic comes from, as some growers feel very strongly that "a herm is a herm".
From my own experience, feminized seeds that I have created myself will turn male with about as much provocation as the original parent would.
The only feminized seed that I have gotten from a breeder is Blue Cheese from Barney's Farm. I have been trying to make some seeds with it, but even though it was popped from a fem seed, and I have done just about everything conceivable to mess with it's feeding, watering, and light schedule, it won't make pollen sacs for me as of yet.