We are currently trying out AVRO although we're not being very fancy with it.
We're actually trying to build on top of this client code right now:https://github.com/ExactTargetDev/kafka.git
Out of all the repos I looked at it seems to show the most promise and had some work within this year. We're primarily worrying about this for producers and it seems to work well enough for that.
I'd love to see anything for 0.8 as I think that's going to be close in our future. Do you have anything out there now?