The PyCon live stream we did was a pretty good case study. We pretty much hacked together a pub/sub system on top on BLPOP, so it'd be even easier going forward. Hopefully we'll open source it :/ (we've been meaning to for a while now...)
I don't have any numbers, but during Guido's keynote (which was twitter powered) tweetfall was too far behind live, so they switched to using our feed. We frequently got comments that we were updating faster than twitter.com was :)
StudiVZ and Craigslist are using multiple Redis nodes as a fast meta-data store. The Boxcar push notifications app for iPhone is Redis-based. Engine Yard is using Redis in multiple ways. Github as messaging system. I'm also aware of many other istalls in a lot of companies, what I'm loving about how things are going with Redis is that's considered more and more like a facility you use every time you need it so it's a tool in many companies. It's a bit hard to track the usages often.
