At the Amazon RDS FAQ there is the question "What is a database instance (DB Instance)?".
The entire answer (as of mid-June 2012) is:
You can think of a DB Instance as a database environment in the cloud with the compute and storage resources you specify. You can create and delete DB Instances, define/refine infrastructure attributes of your DB Instance(s), and control access and security via the AWS Management Console, Amazon RDS APIs, and Command Line Tools. Multiple MySQL databases or SQL Server databases (up to 30) or Oracle database schemas can be created on a given DB Instance.
The last part of that quote, "Multiple MySQL databases or SQL Server databases (up to 30) Oracle database schemas" I interpret to mean that you can have an "unlimited" number of databases on an RDS MySQL or Oracle instance but only 30 databases on an MS SQL Server instance ("unlimited" meaning not limited by the RDS infrastructure itself).
This was asked in the Stackoverflow question Does Amazon RDS support multiple databases per instance?. The answer quoted an older version of the FAQ.
What I am looking for is an Amazon document that clarifies this question, or else someone who has experience using Amazon RDS who can attest what the situation actually is.