I just heard about this site today on Twitter. I'm sure this isn't the only site that helps you share ebooks, but it's the first that I've heard of. While I don't have any ebooks from B&N, I do approve of the principle.
Edit: They've had to change the name to Bookfriend.me
But they do have a couple curious twists on their site. The ask you to buy ebooks through them. They get a cut, of course, but I'm hoping that they are planning to work out a better account integration with B&N. It would be nice if they could pull from B&N a list of ebooks you bought. You could go down the list and check off the ebooks you want to lend. Wouldn't that make it easier for the user?