Can I prevent SMS from showing on the Lock Screen in iOS 4? I have some private SMS messages I want to hide. The problem is that they are displaying on main screen even when it's locked. So it was useless using password because it's displayed even with the lock.

How can I make SMS messages hidden on locked screen?
Go to Settings > Messages. There's an option "Show Preview" there. If you toggle this to off, it will stop showing SMS previews on the lock screen. This will also disable the alert popup when a new SMS arrives and the phone is not locked.