Shape Dynamics won't stay off even when the option is locked with Wacom Bamboo in Photoshop CS5 Extended. After disabling and first line is drawn the setting is on again. I've thought that it could be decided by the user but it seems the program has it's own mind.
I have had the same problem. When you have selected a brush, at the top menu bar over your main window, NOT in the brush menu, there are a selection of controls. You will see the icon of the brush with the toggle, Mode, Opacity, Flow and then an airbrush icon, and after that an icon that looks like a circle with a pen stuck in the middle. It is a toggle and if it’s dark it’s turned on. Make sure it’s off. It’s called tablet pressure controls size. It overrides whatever settings you put in the brush control panel. Hope this helps.