Where does a published RDL file sit?
It is not a file on the server. It stored as a BLOB in the ReportServer database.
(In the Catalog table to be precise on SSRS 2005)
Extended Answer
This page shows how to retrieve this using T-SQL
http://mscrmuk.blogspot.com/2009/05/reading-rdl-definitions-directly-from.html
Jeff
Update: here's the relevant query, in case that blog post disappears in the future:
select convert(varchar(max), convert(varbinary(max), content))
from catalog
where content is not null
Following query specifically provide the report's blob
SELECT Name, convert (varchar(max), convert (varbinary(max),[Content])) AS ReportRDL
FROM [dbo].[Catalog] where TYPE =2