Ok guys I found a solution to the screen lockout - -Power Down problem on my Samsung Galaxy GT-I9000 running Android 2.3.3 (Gingerbread). It took a lot of hair tugging and swearing to get there.
Symptoms: Screen was timing out causing the phone to power down completely. Also a third part screen lock widget was having the same effect when pressed.
Short Term solution: Take Battery out, reboot phone etc etc
Resolution: Go to Settings > Display and untick the option 'Power Saving Mode'. Not only did this resolve the power down problem but my camera software started working again ;)
I think there is a bug in the power saving process that continually runs in the background. Not sure what this is called. I think it gets confused by the screen timeout and causes an operating system exception.