Few unsupported features in 2008 Express Edition … 
- Remote data sources are not supported. Reports that are hosted in a SQL Server Express report server must retrieve SQL Server relational data from a local SQL Server Express Database Engine instance.
- TIFF (Image), XML, and CSV rendering extensions are not supported.
- Custom authentication extensions and custom role assignments are not supported. You must map existing Windows domain user and group accounts to predefined role definitions.
source: Features Supported by Reporting Services in SQL Server Express
more info on SQL Server 2008 family here
Technorati tags: SQL Server, Reporting Services
