at least in Blender 2.48a I experiece following thing:
If I have more than 2 cameras in the scene, after some time I cannot change active camera anymore, no matter how many time I press Ctrl+Num 0. What helps is just to delete N-1 camera and put them in the scene again. I don’t thing it depends on a system, I had it on 3 different PCs.
Any suggestion what to do?