I found that using a sim that does not get service (overseas carrier, no roaming agreements..) caused my phone to keep searching for a usable carrier, and drained my mobile quite quickly.

This was before iPhone, but still an observation - that maybe still holds true

First thing would be to try swapping the sim. It does seem that some SIM cards on some networks drive iPhone crazy (for unknown reasons). So swapping the SIM could troubleshoot that. There may be some setting you need to look into as well since there are many things that effect battery life. You could also have a drain if your phone is connected to WiFi or the push/fetch settings on your phone are activated.Also turn off push for all mail accounts. Take a look at those settings on your phone . Please do not forget about blue tooth devices as well as those connections will drain battery life as well. Also , watch for any apps that are using fetch etc. since they can drain your battery in a hurry. So the way I'd proceed is to

1.Shut down the phone, left it off for about 2 mins, and then restarted (idea is to close down any app that might have crashed and is draining power)

2. Charge the phone to 100%

3. Set location services to off, Wireless on, 3G on, push off, Fetch Manually

and see what happens. Good luck and let us know....