I've searched the web and browsed the RDS docs, yet I can't seem to find an open connection limit.
For what it's worth, I'm planning on using the new Postgres flavor of RDS, but I suppose an answer from the mySQL side of the house would be acceptable as well.
Thanks!
If you setup a PostgreSQL RDS instance you can query for max_connections:
select * from pg_settings where name='max_connections';
max_connections is set to 5696.