I've long since given up my hate of MS. I have a MacBook, an aging PC that I will soon be replacing XP with Ubuntu, a media PC with Win 7, and two Android devices (including the Nook Tablet). Sure, there are plenty of things I dislike about the Windows platform but as a company, Microsoft has long since shed its "Big Bad Wolf" clothes. They are not the untouchable monopoly they once were.
First, to clear some misconceptions, Windows 8 is not exclusive to x86. They will support the ARM architecture as well, which would include the NT. Second, Windows 8 appears to be more a convergence of PC/tablet functionality; a tablet version would give you much the same look and feel as the PC version but, like iOS vs. OS X, the two will not necessarily offer the same capability for running software. You should be under no illusions that you will be productive with your NT as you would a PC. Third, no chance MS will force this on existing Nook Tablet users with root or with a custom ROM. As with other investments, MS was looking for a viable, ready-made hardware platform that was struggling so they could easily work their way into the tablet marketplace.
Potentially cool things about this - more options for OS. You may hate MS but no biggie - just keep Android. You like MS? You may have an opportunity to place Win8 on your Nook. Also, MS will look to have a feature-rich platform - I wouldn't be surprised if they coax B&N into exposing Bluetooth functionality (with the undeniable limitations in range) or expanding peripheral offerings.
Should you expect huge changes with this? No, not any time soon. Should you be worried? If you like B&N, likely no. This could be the move that saves them as they struggle against the likes of Amazon. The Nook Tablet is a fantastic piece of hardware but don't delude yourself into thinking they can take over the Kindle Fire in their current state. B&N has a pitiful app marketplace, rotten support for their hardware, and klunky software. MS could help that, despite what you think of them as a company.
Abandon the Nook if you so choose - you will have no effect on MS. Honestly, though, I'm more disgusted with the culture of Apple than MS at this point (even though I like my Mac and OS X).