I don't think you're supposed to use flash photography on the dark rides, mainly because it bugs other people who are trying to enjoy the ride. CMs will actually tell you to stop if they catch it on their monitors.
There are a few rides on which you can get away with it, though, without drawing too much attention to yourself. I have taken pictures on Roger Rabbit (you have to have steady hand!), Splash Mountain (only on the outside parts; protect your camera on the drops!), Alice in Wonderland, Snow White, and it's a small world. You could probably get away with it on Pinocchio and Mr. Toad also. Another way to get picture of inside the rides is to buy postcards.
DO NOT try to take pictures on Pirates of the Caribbean or Haunted Mansion. People inevitably try this, and it makes the CMs and other guests really mad.
I hope this helps. Good luck!!!