Hi Emily,
I think that every component of SQL Server requires a full SQL Server licence for every server that it is installed upon.
That is to say that if we were to take a single server and install upon it the database engine, integration services, reporting services and analysis services - we would require one SQL Server licence to cover the whole suite.
If we were to take 4 servers and onto each install one individual component. That is to say the database engine on the first, integration services on the second, reporting services on the third and analysis services on the fourth - then we would require four full SQL Server licences.
When you scale out reporting services this rule still applies. So a scaled out deployment with the content database on one server, and the reporting services application servers on two separate servers will require a total of three licences.
Not sure if Microsoft certifies an SSRS 2008R2 report server with a 2005 content database.
But I have installed SSRS 2008 R1 in a scaled out deployment with two 2008 R1 application servers and a 2005 database server housing the content database, and this works fine.
I did check with Microsoft before I implemented this configuration and they confirmed that this was supported. You may like to check again for R2 though.