User-settable properties in Reporting Services
SQL Server Reporting Services reports have a number of properties that can be obtained via the GetProperties web method.
The full list of properties is documented in SQL Books Online in Report Server Item Properties. What it fails to mention is that some of these items are read-only. Try passing them in via CreateReport or SetProperties and you'll just get an exception for your trouble.
By trial-and-error (mostly error!) against an instance of the SQL 2005 Reporting Service Web Service I discovered that the following properties are user-settable: