Engine Yard is moving well beyond its roots. On August 23rd, the company signed an agreement to acquire Orchestra to add PHP to its Platform-as-a-Service (PaaS) offerings. Now it’s announcing plans to add to its database stack.

In a post last week, Engine Yard’s Ines Sombra outlined plans to upgrade the MySQL implementation, and expand the database stack to PostgreSQL 9 and MongoDB.

According to Sombra, Engine Yard is going to be upgrading MySQL to 5.1 and 5.5. (You can find the current supported stack on the Engine Yard site.) PostgreSQL 9 is in a limited alpha preview for Engine Yard customers, with plans for a public beta shortly. MongoDB is not yet in alpha, but Sombra writes that Engine Yard has partnered with MongoHQ and MongoLab.

Engine Yard has its work cut out for it keeping up with Heroku, which was acquired by Salesforce.com in December 2010. Heroku has been busy as well, adding Node.js last year and Clojure recently. (Heroku also supports Ruby, of course, and Java.) For datastores, Heroku supports a wide variety – MySQL, MongoDB, CouchDB, Memcache, and PostgreSQL.

After picking up PHP, PostgreSQL, and MongoDB, where do you think Engine Yard should go next?