I noticed recently that the TechNet documentation titled Considerations when using antivirus software on FF Edge Products available here contains an error within the Forefront TMG section.
The process exclusion for ReportingServicesService.exe is defined with a path of:
%ProgramFiles%\Microsoft SQL Server\MSSQL10.ISARS\MSSQL\Binn\ReportingServicesService.exe
and this should actually read:
%ProgramFiles%\Microsoft SQL Server\MSRS10.ISARS\Reporting Services\ReportServer\bin\ReportingServicesService.exe
I have provided feedback on the page, but until this gets changed, you may want to update your existing deployments with the correct process exclusion path.
Hope this helps!