I’m not sure there was every any real conviction that the Android Market would eventually make it to the Nook Color (even with the 2.2 update coming in January), but nevertheless, Barnes & Noble has made it clear that no, there’s no chance. The Nook Color will have apps made just for it (the SDK was recently released), but beyond that, apps are verboten.
Of course, the device has already been rooted and if you’re feeling brave, you can go through the many steps required to run Robo Defense. I’d wait until there’s an easy rooting tool, though. Too risky.