Interesting ideas. I've never used stumbleupon, but that does sound pretty similar. I'll check it out. If it adds a lot of complication, just do it a simpler way. The main thing is that the chat is very obvious and easily accessible.

Perhaps this is more of a long-term idea, but it would be interesting if people could instantiate a chat window in the comments. Like if a hot thread gets a couple responses deep, someone could type the text "$!NEW_CHAT" or something as their comment, and it would replace that comment with an interactive shout/kiwi IRC terminal. Perhaps also even automatically creating a unique IRC room specific to that sub-sub-thread (named with a structure like "#SubredditName_ThreadName_SubTheadNumber"). Then anyone could start a live chat about anything, anywhere.

This ability to summon live chat in comments could be in addition to having the sidebar/stickied subreddit chat. I think that would be extremely cool and powerful. But that's sort of on the wish list, no rush