Lock the device
Verify that you have set a BlackBerry® device password.
On the Home screen or in the application list, click
Lock.
To unlock your device, on the Lock screen, click the trackball. Click Unlock. Type your device password. Press the Enter key.
In reality all you have to do is start typing your password from the home screen. If you have your dial from home screen option set to off, so that home screen shortcuts do work, you can hit K to lock the device.
Lock the keyboard
If you do not set a BlackBerry® device password, you can lock your keyboard to avoid making calls and pressing keys accidentally.
On the Home screen or in the application list, click
Keyboard Lock.
To unlock the keyboard, press the asterisk (*) key and the Send key.
In previous versions of the RIM OS dating back to atleast 3.6, if the device was locked, you could select 'Unlock' from the lock menu and it would actually unlock the device. It no longer does so, and people are upset by this. If you only seek to prevent accidental keypresses, see the following.
Turn on standby mode
If you are not on a call, you can turn on standby mode to avoid pressing keys accidentally when you are not using your BlackBerry® device.
Hold the
Mute key.
To turn off standby mode, press the Mute key again.
This will allow incoming calls and emails. This is not a security feature, but it does prevent accidental keypresses.
I personally don't understand locking the device without a password. All rants and raves accepted.