My first though after reading your comment was that there's probably a very small number of people that read HN and support SOPA. After thinking about it a little bit more I realize that my dismissive attitude was inappropriate.

When Reddit goes down there are going to be news stories about it and these will raise awareness about the issue to people who aren't Reddit users. If HN or any other sites join in then they will most likely be mentioned in these articles and to the casual reader this will only make the message stronger even if they aren't familiar with the actual sites. In addition, HN has a huge audience of people who are in a position to blackout their own sites. If HN were to join in then I think that this could be influential in making these people consider more seriously whether they should follow suit which will only further the reach of these actions. At the very least, HN being down could serve as a good reminder that opposing SOPA isn't enough by itself and that if we really care about this then we should be contacting representatives or doing something else that could have even a small impact.

For these reasons, I would also be in favor of HN joining in support.