The SD cards being sent it true, but only partially. The SD cards are put into the devices before startup and then booted from it specifically. Then the FW gets flashed with the information that is on the SD cards. Once the device is flashed/updated with the data from the SD card you do NOT need to keep the card in the device. In fact you should not keep it in the device.
To ENABLE demo mode you have to go to All Settings > Device Information and then press the Volume Up button and tap with your finger just above the volume bar that shows up at the top of the screen. Another menu pops up with a box asking for Demo mode (among other information). If it's checked, uncheck it. But just keep in mind that the demo information is still flashed onto the device and I'm not aware of any way to restore it or erase it.
But once it's out of Demo mode you should be able to deregister the device (has happened at my store one or twice already). But like some others have mentioned, the serial numbers for each demo machine are tracked so it might be flagged as stolen.
Thanks that worked. I just want to try it out. Unlike the kindle fire I'm sure some one will come up with a modded android OS to wipe out the nook os. much like when the nook color first launched. Thanks for the help though