Well this is a surprise. After being one of the first to release a "reader tablet" (ahead of Amazon), with a custom interface and custom app store, it seems B&N has licensed the Google Play Store from Google for its Nook HD and Nook HD+ tablets.
B&N will send an over the air update, that will include not just the Google Play Store app, and all of its 700,000+ apps, but also all the Google Apps you're used to on a regular Android tablet, including the Chrome browser. Nook HD and HD+ are now officially regular Android tablets. This instantly makes them more compelling than the Amazon Kindle tablets, which remain far more restricted in terms of apps, and due to their lack of Google apps, too, such as Gmail, Google Maps, and so on.
"By adding Google Play to NOOK HD and NOOK HD+, we are offering our customers even more great entertainment on our award-winning tablets." – William Lynch, CEO of Barnes & Noble
I believe this is a very smart move from B&N, as lately, Amazon's Kindle, while not as popular as it used to be, still managed to get much more popular than B&N's own tablets. Something like this could at least put B&N back on the map for the Android community, and people may now consider their tablets again.
That being said, B&N's tablets still face very high competition from Google's own Nexus 7, especially with the upcoming Full HD Nexus 7, and a much faster processor than that Nook HD and Nook HD+ have. Not only are the specs much better, but you also get stock Android, the latest version of Android, and updates for another year and a half.
It's going to be tough for B&N to fight against that, and they will need to release a next-gen Nook tablet, too, with some impressive specs. But I don't expect that to happen until next fall, which is when they usually release their Nook tablets.
If you're already a Nook HD or HD+ owner, you should expect the update with the Google Play Store app and Google Apps to start rolling out tomorrow. The update is optional, so you don't have to accept it.