Request for credentials when using a shared data source with Report Builder 2.0 over the Internet with forms authentication

Question

SQL Server 2008 (not R2). I am trying to allow users to create and deploy SSRS reports on a report server, accessible only to users over the Internet. I created my own forms authentication provider.

Now I can connect to the report server via the Internet through Report Manager or Report Builder 2.0. What I would like to do is to provide the user with a common data source that he can use in reports. I created a data source with credentials stored on the server. Now I can create a report and add a shared data source. However, when I try to add a dataset, I will be prompted to enter credentials for the data source. If I have access to the database server over the network, I can enter the credentials and everything will work. However, when working over the Internet (the db server is not accessible from outside the firewall) it fails with a "Login Error".

I think he is trying to connect directly to the database server when I do this, instead of connecting through the report server and using a shared data source. If I use the general model on the server instead of the data source, it works fine, but I would like to use the flexibility of using the data source.

Is it possible? If not, does anyone know if it is different in R2 / RB3? I do not have access to R2 for testing.

Thanks in advance,

Josh