Is your intent to have your entire workspace displayed on the Wacom or just the window with the image you are working on, with all of the toolbars and palettes on the MBP display?
I would think you want just the window that contains your image on the Wacom, to maximize image real estate, and everything else on the MCP display - then use keyboard shortcuts to change modes, tools, etc so you are not mousing back and forth across displays. To do this, follow René's suggestions about arranging the displays in the System Preferences. If you are using PSCS4, you may have your application in Application Frame mode - you may need to uncheck this option to go to the traditional windowed mode, then you can drag that window over to the Wacom display. You can also drag any other PS window over too, like the tools, etc. Lightroom is probably different, slightly, as I believe that it has its own way of dealing with multiple displays internal to Lightroom.
Good luck! let us know how the Cintiq works for you, as that is a pretty cool concept.
Kirk