I've had my iPhone for 2 weeks and updated to the latest firmware version as soon as it came out. I've had 3 occasions where I have taken my iPhone out of my pocket to find that it's powered itself off. It doesn't appear to have locked up as it powers on as normal. The battery is always fully or well charged.
I've posted earlier that I've pretty much narrowed it down to the Skype app, and still do. At least on my device. When I format my phone and re-install one app at a time, it's the Skype app that will cause the next crash within 24 hours. Every time.
I have a MORE case and use skype-out frequently, it let's me finish any conversation ranging from 10 mins. up to an hour, but as soon as I hang up and exit the application, the phone shuts down...