I don't think this has been addressed here, and I am also annoyed by it. It didn't start happening on my nook until a week or two ago (I don't have wifi on all the time, so I can't give a specific date), and I've been using a nook since the first one was released. I bought a nook Touch a couple of months ago. This is the first time B&N have ever done something like this and it's VERY annoying and honestly makes me feel mildly violated.
If anybody has found a way to opt out of this sample book scheme, please post it so we can do the same!