If anyone's interested, I've recently released the seamless_database_pool plugin which adds an adapter for using a master/ slave database cluster with ActiveRecord. The emphasis of the code is to make it trivial to add to an existing Rails application. You don't need to refactor your code or change how you use ActiveRecord. It can also ensure consistent connections for a request to eliminate those sinister bugs that crop up only every 1000 requests due to synchronization delays.
If your running into issues with scaling your database you may want to check it out.
http://agilewebdevelopment.com/plugins/seamless_database_pool