Is it possible to use an EC2 RDS MySQL instance as a slave to an external master?

I have a master MySQL instance running on our local network, but I was wondering if I can get an EC2 RDS instance to replicate the master or is this locked down by amazon?

Apparently this is now possible, however it is not fully "supported". Amazon will now allow you to replicate an RDS to an external slave, and replicate to an RDS from an external master, however their disclaimer suggests that they don't intend for this to be a permanent part of your server setup.

I have personally not tested this, but I plan on using it to help migrate off of some old non-AWS servers in which I host various databases and applications.

Given your question the below AWS doc will best apply.

Replication to RDS from external Master: http://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/MySQL.Procedural.Importing.NonRDSRepl.html

Proof of concept on Sönke Ruempler's blog post (RDS to External Slave): http://www.ruempler.eu/2013/07/07/replicating-aws-rds-mysql-databases-to-external-slaves/

This is still not supported as of February 2011, see RDS instance as a replication slave:

Non-RDS to RDS replication is not currently supported [...]. However, we will log your interest for our future road map planning.

You can do this with Tungsten Replicator, which is an open source replacement for MySQL native replication. It now supports replication from a MySQL master to an Amazon RDS slave. For more information, please see the following blog article:

http://scale-out-blog.blogspot.com/2013/01/replicating-from-mysql-to-amazon-rds.html

Cheers, Robert Hodges (Tungsten committer)