I don’t know how many have heard of or used Drizzle , the MySQL engine optimized for cloud and net applications, but there seems to be some activity (from Marcus Eriksson) around creating Drizzle replication to different NoSQL stores: Project Voldemort and memcached  or Cassandra .
Leaving aside the technical details — which are definitely interesting , the solution using the Erlang AMQP  implementation RabbitMQ  — I think this replication layer could represent a good basis for SQL-NoSQL hybrid solutions, which is a direction we’ve mentioned before: Introducing the Oracle-SimpleDB Hybrid
It would be interesting to hear other stories from those that are investigating the NoSQL hybrid solutions.
And while we are at MySQL engines, I thought I should also mention this question from Ilya Grigorik (@igrigorik):
anyone try or using TokuDB ? drop in MySQL engine using fractral trees, claims 10-50x over InnoDB, etc.