I have to believe Apps that do not remove themselves from memory when closed are in fact causing additional battery usage. If an App is constantly attempting to connect to the Internet, or refresh it's status in memory then for sure it is using battery power. Recently an App was released on the B&N App Store named, "My App Manager." This handy App allows you to see which Apps are active in RAM and the opportunity to "kill" (close) them. I recommend you purchase this App then after using Texas Holdem Poker close it with this App. Let us know if your battery drain issue is resolved.
I downloaded Android For Nook last week, and one of the most helpful parts of the CM7 system that it installs tells you what % of your battery each app has used. I wish BN would add that feature, or at least an app to track it. You would think that it would save them support costs from people thinking their NTs or batteries were defective.