I've taken pictures with my own camera with ET (I just took one on Dec. 11.) No one said a word. I also use my own camera for other characters like with Spiderman or Barney. Even though they have a camera set up, I've never purchased the picture, just used my own. Sometimes, I let them take a picture but I always tell them that I'm not planning on purchasing it. They just give me a slip of paper with the photo number printed on it.
The only time I've seen anyone told they couldn't use their own camera was for people who were taking a picture of the ride photos (like at photos on the monitors at MIB, Mummy, and Hulk.) If you are refused by anyone from taking a picture with characters, please let guest services know.
Personal cameras can't be used in the queues or on the rides but, with characters, it is fine.