Is there any overlap between the work done in Gizzard and the other popular MySQL forks (Percona, MariaDB, Drizzle) or are each of these solving different (incompatible) problems?
Gizzard is sharding framework that sits on top of, as a separate layer from, MySQL (or any other storage backend, like Redis). It's open sourced right now at https://github.com/twitter/gizzard. Twitter's MySQL fork for now is just a way to get in bug fixes and extra features that are needed but not available upstream.
Yup! And our changes are quite complementary to Gizzard, since it's one of the larger use cases at Twitter for MySQL. More to come in the future to better support big Gizzard systems!