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?
Actually, there is no text log file for Reporting Services, at least not for the execution logs. Please se http://technet.microsoft.com/en-us/library/ms157403.aspx and http://technet.microsoft.com/en-us/library/ms155836.aspx for more details. The first link also provides more information about the other log files.
Assuming you installed SQL 2008 to the default location: C:\Program Files\Microsoft SQL Server\MSRS10.MSSQLSERVER\Reporting Services\LogFiles
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On SQL Server 2005 the log files (on my installation at least) are in c:\Program Files\Microsoft SQL Server\MSSQL.3\Reporting Services\LogFiles