Did the service centre run a software update? If they didn't you could try that yourself by using either PC Companion or Sony Ericsson Update Service.
The only other thought is do you have a cover on the phone that may be pressing the side button and triggering the camera to come on?
If none of these things work then I suggest you contact Sony Ericsson direct just find your local number from this list...
Sony Ericsson - Corporate - Common - Contact - Phonelist
