AE CS5.5 3D question

You can set the default keyframe interpolation in the general preferences by checking or unchecking "default spatial interpolation to linear" and AE will always create new keyframes with that interpolation.

Bezier handles are only visible on selected keyframes in both the graph editor and the comp window, so unselected keyframes may appear to be linear. Also, bezier handles can be very hard to see in the comp window, even on selected keyframes, when the animation path is straight instead of curved because the handle perfectly lines up with the path and the only indication it's there is a slightly larger dot along the path that indicates the end of the handle.

HTH