What Trello accomplishes could also be accomplished in Campfire without needing to keep a browser window open. There's also Yammer, to some degree.

Obviously Trello's approach is different, but my point was there are other apps that have simpler approaches to the same problem.

http://agilezen.com/ is very close, and actually has a bunch of other useful features too. Although I've got to say that trello is a lot slicker than agilezen.