If you plan to use Report Viewer WebPart (from SQL Server Reporting Services) in your WebPart pages dynamically (i. e. use this WebPart as connected WebPart - receiving Report Url): DO NOT FORGET TO INSTALL THE WP PACK GLOBALLY (-globalinstall option)!!!
Refer to: http://technet.microsoft.com/en-us/library/ms159772.aspx
STSADM.EXE -o addwppack -filename "C:\ Program Files\Microsoft SQL Server\90\Tools\Reporting Services\SharePoint\RSWebParts.cab" -globalinstall
Otherwise you will get always an unexpected error on exit edit mode.
Enjoy!
