I will admit, I am a little nervous about all of the ebooks I've gotten, but I would like to think if something happened that B&N went away that somebody would do something to allow me to keep my e-books.
That "something" already exists, though with B&N you have a much lesser fear than with other ebook vendors. For Amazon or other ePub retailers, you're dependent on Amazon or Adobe sticking around; B&N you just need a computer you can reinstall the reader on.
If you're going to take advantage of "something", though, best do it while you can instead of waiting until the last minute. You never know when it'll be too late to (re)download them from B&N.