To second what everyone else has already said, make sure the park are OK with you filming first. Smaller parks and those in Europe generally seem to be more accommodating, whereas bigger park chains (especially in the UK and USA) seem to be more risk averse.
I've shot a few POVs over the last few years, but only when visiting parks as part of a CF group, during exclusive ride time and after making sure that the ride operators are OK with me doing so.
Make sure you know how you're going to secure the camera too. Regardless of whether or not you're mounting a camera (like some of the people I know) or shooting hand-held (like I do), parks are going to want to know how you intend to prevent the camera from going flying mid-ride. Just holding on to a camera phone and hoping it doesn't slip out of your hand isn't good enough. Remember that a flying camera could seriously injure or kill someone if it collides with somebody at high speed. Don't be an idiot. Plan ahead.