Permission to view reports for SSRS 2008 in SharePoint 2007 integrated mode

we know one of nice things about SSRS in SharePoint integrated mode is that user permission is managed by sharepoint site or library, in other words, to grant a user the permission to view report is as simple as to grant a read permission to sharepoint site or library where reports sit. It is very user friendly!

But one exception is, if you log in as sharepoint farm account or application pool id account, even though both accounts are marked as "System Account" and have access to absolute everything, it seems SSRS is not aware of them. You will get the following error when trying to view reports from sharepoint:

The permissions granted to user 'SP\spFarm' are insufficient for performing this operation. ---> Microsoft.ReportingServices.Diagnostics.Utilities.AccessDeniedException: The permissions granted to user 'SP\spFarm' are insufficient for performing this operation.

[update] this problem only  occurs to sharepoint 2007 when kerberos is enabled for both sharepoint and RS, and data connectin uses window authentication. Sharepoint 2010 has this problem fixed.

The workaround is to add them explicitly in site or library, and the problem is gone.