Apple knows about locked screen calling in iOS 4.1, will be fixed with 4.2 in November
Wired.com reported that the current iOS version (4.1) has a major security loop which lets one make a call even when the phone is locked all you have to do is press a sequence of buttons and voila your supposed to be locked iPhone is now fully ready to make calls, this is especially risky for when you think your phone is secure and someone is using your phone. Apple has been made aware of this scenario and they are working to fix this issue in the upcoming iOS 4.2 which is scheduled for November.
This is quite a dangerous trick I highly recommend that all those who are on iOS 4.1 should not leave your phones un attended. This is a major security risk and anyone who knows about the security loop can take advantage of your lack of knowledge of this loop. Until iOS 4.2 releases every locked iPhone running iOS 4.1 is a potential contact list ready to be cracked open.