Hi Samwardill
We have actually had exactly the same problem with about 10 of our N73s, mainly ones used by bands that get thrown around a lot. Basically it is a physical problem with the little prongs and lens cover door. After a while they can become misaligned and no longer detect when you open the door (to start the camera you need to hold down the camera shutter button for a couple of seconds instead).
The only solution we have found is to replace the door/battery cover with a new one. Nokia may do this for you under warranty.
I have also replied to you over on the ShoZu forum.
Best,
Mark (ShoZu)