The most relevant google result seems to indicate that in order to access the logs we have to depoly our own log tables to the database and make Reporting Services write to it.
Simply put: surely there must be plain text log files for Reporting Services? If yes, where are these files stored?
Assuming you installed SQL 2008 to the default location: C:\Program Files\Microsoft SQL Server\MSRS10.MSSQLSERVER\Reporting Services\LogFiles