I had no problem registering my Nook at home when I first got it. But when I had to do it again several months later after a factory reset it wouldn't work. The set up was exactly the same as the first successful time and I quadruple checked and reset everything but no dice.
Others this has happened to have said they were successful using another wi-fi source. So I wound up registering while standing outside a McDonalds using their free wi-fi. It took about 10 seconds. Strangely after I registered I was able to connect at home.