For what it's worth, Amazon and BN both allow publishers the option to sell non-DRM ebooks in their markets, so we also have to convince publishers that it's in their interest to publish DRM-free ebooks.
Nooks read epub, an open format that iBooks, Sony Readers, Kobe and most other ereaders use. If you can find an epub ebook, you can read it on the Nook. The DRM that Nooks use, when it is used, is from a DRM from Adobe that's pretty widely used, so it's by no means exclusive to Nooks.
Not so the Kindle. Amazon created their own format that only Kindles read, and purposely stayed out of the epub development that BN, Google & Apple are part of.
To find out lots more about DRM and related issues for ebooks, I recommend Cory Doctorow's book "Content." You can download it as a free epub ebook, or read it in html
here.