I saw this post where someone is asking about the migration path for reporting Services.
This has also come up in conversations I have had so here is my take on it:
MSRS 2000 reports can be uploaded to SSRS 2005 and will run correctly.
SSRS 2005 reports are not supported on SQL Server 2000 reporting services. It may be possible to edit the XML name space in the .RDL file and get simple reports to be backward compatible but do it at your own peril.
Visual Studio 2005 (Business Intelligence Suite) cannot deploy to SQL Server 2000 Reporting Services.
Visual Studio 2003 can deploy to SSRS 2005 but you cant get the benefit of the new SSRS features.
I hope this is correct.
PLEASE NOTE I HAVE NOT TESTED OR CONFIRMED THESE HYPOTHESIS WHICH ARE MY UNDERSTANDING ONLY.