for me, the comment i'm replying to shows a red dot, just to the right of the up/down arrows. i've seen a couple of other comments with such a dot. anybody know what it means?

It means the comment has reached the "good" threshold and has 5+ upvotes. See: http://news.ycombinator.com/item?id=846123

pg's experiement http://news.ycombinator.com/item?id=844979

If you didn't know that, HN probably isn't the right place to start learning it.

It rolls so quickly, I would find it difficult for anyone who doesn't know this to grasp the fact. It took me a second, and I'm awesome at basic mathematics. BTW, diameter here = 1.

Anyone else thought that π(n) was referring to a graph of the density of prime numbers? :P

It took me three watchings to figure out what they were trying to demonstrate. Between the colors (not to harp on the color blindness thing, but those color choices were dreadful) and the speed of the animation, its relatively difficult to even understand what is happening the first few times through.

I agree. The version here http://en.wikipedia.org/wiki/File_talk:Pi-unrolled.gif appears to move slower. I think it's better.