Hi all, there is no quick way to one button lock the Treo 650, but I have found a button re-assign that helps a lot. Press the home key (near the green call button) scroll down to preferences, in preferences locate buttons, at the very bottom is a way to set the side button (or any other button you don't use, I personally use all the other buttons) and select 'security'. With that set you can press (and hold) the side button and it opens the security app (where 'Lock & Turn Off...) button is, now just press the Lock & Turn Off... and it is locked
I can have the phone locked quickly using that method.
Hope it helps you out!
Have a terrific day!