If you turn off dialing from the home screen (press the green phone key/blackberry key/options/general options/ dial from home screen) you can press the "k" key and it will lock. There are also more shortcuts. To dial in that mode, you would have to press the green dial button first.
Posted from my CrackBerry at wapforums.crackberry.com
I set my last home screen short cut to the lock app. So I just flick the trackball to the right and press it and it locks. really fast and don't have to use a conveniance key for it
Posted from my CrackBerry at wapforums.crackberry.com