Now, modders love this stuff because it gives then the opportunity to turn a $259 ebook reader into something far cooler and multi-featured. On the flipside, I'm pretty sure that B&N hate this sort of thing because it wants you to be buying books with the device, not freeloading an internet connection on their coin. Without a doubt this sort of thing goes against the terms and conditions, invalidates the warranty and risks the device from being exiled from the B&N mothership, but that doesn't stop people doing it, especially if they get something for nothing.
Note: I expect B&N to release updates to try to make the modding more difficult, or at least make freeloading the cellular connection harder.
How many people exactly are going to mod their nook? Even the widely popular iPhone isn't as has jailbroken as widely as some people suggest. The same is likely to be the case for the nook (unless the lure of free internet gets around), because the majority of buyers won't feel like taking their device apart and invalidating the warranty.
I like following these kind of mods because it highlights the ingenuity of some folks, and the lengths they are willing to go to in order to find out what makes things tick, and how to make it tick a little differently.