Can someone explain last year's reddit exploit to me?

Question

Last year a user managed to inject arbitrary javascript into reddit's markdown syntax. Can someone explain how this was done and how I can test whether my site is similarly vulnerable?

Answer 1

Blog entry on the exploit:

http://blog.reddit.com/2009/09/we-had-some-bugs-and-it-hurt-us.html

The patch that fixed it:

http://code.reddit.com/changeset/1f1f0606f5b6bf14a0db55a28cfd03e1e42e3550