Nook Tablets will be automatically updating to version 1.4.1 over Wi-Fi all this week, but if you're one of the people that likes sideloading non-B&N apps, you won't be happy about it. Barnes & Noble says only that the new firmware simply "provides minor system enhancements," but there are reports from both xda-developers and the B&N Community that it also disables sideloading apps and removes root access. (Maybe those are the "enhancements.") All is not lost however; any apps you installed before the update will continue to work.
If you don't want to give up sideloading, you're going to face two pretty inconvenient options: avoiding Wi-Fi altogether, or rooting your tablet to disable automatic updates. The people at xda-developers might eventually figure out a way to work around the restrictions of 1.4.1, but you'd probably end up needing to root your device to do that anyway. Keep it simple: just sideload your favorite unsanctioned apps before the update hits your device.