Your prayers have been answered
In After Effects CS4, you can select the position property and go to Animation > Separate dimensions. This will split position XYZ into three separate positon properties. There's also a button for the same operation in the the Graph Editor. Even better, you can join them back together if at some point you decide to do so. Bear in mind that there are some prices you pay when you separate dimensions. Motion paths are way less flexible, and you can't rove keyframes across time. Both things go against the notion of separate XYZ. I think by providing now both ways (joined or separate XYZ position) AE offers the best of each.
If you have a previous version, there's a workaround - you can apply the Separate XYZ position animation preset in the Transform preset category. It will create an effect with separate sliders for X, Y and Z, which internally drive the position property with expressions. It's not as convenient as the new, proper implementation in CS4, of course.
Thank you so much for your help. Unfortunately I'm running CS3 and for some reason I don't seem to be able to apply the Separate XYZ Position preset to a camera. I get the error
After Effects warning: Function Effect is undefined
Error occurred in line 1
Comp: 'New Middle'
Layer: 1 ('camera 3')
Many thanks in advance,
Create a Null object.
Make it a 3D layer.
Apply the preset to the Null.
Parent the camera to the Null.
Use the sliders in the custom effect to animate.