I believe that by design, no this is not possible. RDS instances actually live on EC2 instances that the AWS system creates and maintains for you. You have fairly good admin control over the back-end of the RDS (DB Parameter Groups are good for this) but since you have no OS access you can't reach far enough to configure this, even if you were to run the RDS within your own VPC that has a DC running in it.
If you need a SQL server with full AD connectivity, I believe your only good option within AWS is to run an EC2 instance that you domain, and run SQL 2005/2008/2012 on it.
(By the way, I would not be surprised at all if AWS engineers are thinking about how to pull this off, since SQL as a service that you could easily join to your VPC domain would theoretically not be that difficult and would be a "killer app" of sorts. The problem is it would disallow multiple/different RDS clients living on the same box, so they would basically need to recreate what you could create in EC2 above.)