Like most other tablets that aren't the iPad, Amazon's Kindle Fire tablet (and Barnes & Noble's Nook Color) is powered by Android, Google's open-source operating system. Because the programming code to Android is available for free on the Internet, companies like Amazon and Barnes and Noble can use it to make smartphones and tablets without needing to ask permission.
E-readers like the Nook Color and Kindle Fire only run Android "under the hood," so to speak. Everything on top was made by either Barnes & Noble or Amazon, and was designed to make it easy for you to read and buy books and videos.
Now, some clever people are producing ways of turning your eReader into a bells-and-whistles smart pad. The CyanogenMod team has taken the open-source Android programming code and made a custom version of Android, which can be used as an aftermarket upgrade for smartphones and tablets in the same way that PCs and Macs can be upgraded with Ubuntu. And people on the XDA-Developers message board are experimenting with ways to bring CyanogenMod to the Kindle Fire, while the Nook Color already has well-established ways to turn it into a full Android tablet.
But do you want to do it? Personally, I can't imagine why anyone would want to surf the internet or receive and send email while reading a book. And, there are all kinds of technical drawbacks, it seems.
If you convert your Nook Color, you won't be able to bring it into a Barnes and Noble store for free browsing and in-store specials ... unless you put Android on the microSD card, and keep your Nook Color's built-in software in the internal memory. (The Kindle Fire doesn't have this option.)
And you won't get any tech support from Kindle or B&N if your tinkering goes wrong. It's just as if you opened up your computer and then regretted what you had done.
It's definitely something for techies. Not, in my opinion, Real Readers.
But you can read more of Jared Spurbeck's technical meditations here