The source for HN comes with Arc: http://arclanguage.org/install
More seriously I don't think Arc is a good idea for an HN clone; the lispiness of it all is cute but the way continuations are implemented doesn't seem to scale in a good way in real life. Some form of cursoring like what Reddit uses for pagination is probably better. Plus the news example couples the layout/theme to the UI code in painful ways.
It is kind of funny that a forum like this, with this layout, and that "implementation side effect", written in a similar style in PHP would be utterly lambasted here.