Whenever new devices come out, there are hobbyists who will attempt to have the first to hack the device. This does not mean doing anything illegal but instead learning how the device works and seeing if one can get past any artificial limitations in the device. It did not take long for a group to find out how to "open" the Nook making it a fully usable operating system.
The Nook has two microSD slots in it. One is for external files that you might be bringing over from your computer. The other is the operating system in this case, Android. The Android operating system was artificially limited. If one were to take out the microSD that Android is being stored on and change just one file inside of it, the entire operating system opens up.
Why does any of this matter?
The Android platform is an open platform since it's based on open source code. This means that a device like the Nook might be one that people will buy for the specific purpose of having an open device. Some want to do more with their devices than just what is intended alone. They want to be creative with them and have them run applications and functions beyond the mere artificial limits that are placed on the device.
For some this will become a portable computer that can be used to run basic applications and connect to the internet. Since the battery life last days days as opposed to hours, it gives a significant advantage over netbooks and cell phones which can not be turned on as long.
More than likely though, the cell phone connection will be limited else there will be too much bandwidth being used for free. Something like a data plan could be in the works in the future giving users a way of surfing the internet on these type of devices.