I have added cameras to my composition and normally they affect any 3d layer showing in the composition at that time. They have stopped doing this for some reason which i can't figure so i have set the cameras as the parent layer for the layers i want it to effect. This works when watching it in after effects but when i export my project as an image sequence or quicktime movie the effects of the camera are not exported, it's as if i haven't put one in at all!
It sounds like you're not viewing the actual camera view whilst previewing your comp. Which camera is selected in the bottom of the comp preview window? You can only render using an actual camera view. Any of the additional camera views (top, bottom, left, right, custom view 1 2 &3) are not useable when rendering. Make sure the viewer is set to "Current camera".
Indeed, I think Andrew got it in one. Also make sure you are not using multiple cameras at the same time. In that case AE will only use the topmost one in the timeline. To render multiple cameras and get synchronous clips, you will then have to duplicate the comp a number of times and render each of them individually with the respective cam.