Hey Josh,
After looking at the site for a bit, I have to admit that I'm not entirely sure. There weren't any links saying "powered by" - which might provide the obvious answer.

At the very top there is a line in the head:
"<script type="text/javascript" src="/static/thathigh.js"></script>"

Looking at the source code, it seems like they are just using plain old Javascript, HTML and CSS. I'm sure they also have to use PHP to store all of the comments. I wouldn't be surprised if there is a plugin that could create something like this on wordpress (or possibly one of the other blogging platforms, like tumblr).

Hope this answers your question.

Lostarts is correct. It is that js script in conjunction with jquery.

Actually, a pretty cool use of jquery too. The submit button triggers a js function that checks the info typed into the text box to ensure it is not too short or long, and then generates the code that places the comment on the page, puts in the correct div and generates the addthis code you see in each of the comments.

Pretty cool.