I've been thinking about this for a few weeks now. We currently run a Percona XtraDB cluster with HAProxy installed on each of our application servers. While this provides some good redundancies, the HAProxies don't know how busy the DB nodes are and it won't continue to scale well.
To that end, we're looking to break up the Percona XtraDB cluster into a more traditional Master - Master setup with multiple slaves under each master and HAProxy installed on a pair of servers with a VIP handled by CARP (technically ucarp).
But I'm looking towards the future of MySQL 5.6, the new GTIDs replication, and how to setup HAProxy for that configuration. To that end, in the next week or so I'm going to setup some virtual machines to test this all out with.