Guest Principal Assigned To Datareader Role Does Have Access

Hello (help),

In SQL2000, when the Guest account was assigned into a role, such as db_datareader, then querying across databases worked just fine.

Specifically:
I have a Report Writer application that connects to the SQL Server with a login (ReportRunner) that actually has very limited permissions on a database.
The connection is then set (sp_setapprole) to use an Application role (App_RR) that has the necessary permissions.
The report-writer app calls a Stored Procedure that gathers data from several other databases (on the same SQL instance).
In SQL 2000, accessing these other databases was done through Guest - we assigned Guest to the db_datareader role. All worked fine.

We've just upgraded to SQL2005: reports started failing. It seems that although guest is assigned to the db_datareader role, the permissions for Guest don't allow selecting from tables via the db_datareader role: we've had to GRANT SELECT TO Guest specifically on the tables necessary for the report.

Is anyone aware of a design change withing SQL Server such that the Guest principal's roles are disregarded when assessing permission? Is there a new and better way to structure the permissions?

Thanks in advance for your help.
Mark Starr