Thanks - lots of handy links, including one [http://www.seomoz.org/blog/reddit-stumbleupon-delicious-and-...] to "Reddit, Stumbleupon, Del.icio.us and Hacker News Algorithms Exposed!"

The formula for HN (as of July '08): (p - 1) / (t + 2)^1.5 where: p = votes (points) from users. t = time since submission in hours.

p is subtracted by 1 to negate submitters vote. age factor is (time since submission in hours plus two) to the power of 1.5.