Portal Users Are Unable to View 'User' Object Fields in a Custom Report Type

When you employ a Custom Report Type as the basis for a report, you have the ability to select spanning fields that expose fields from the User table when crafting the report (e.g. Created By: Full Name, Last Modified By: Full Name).  These fields can then be included as columns or used as filter criteria in the report. 

However, if you expose this report to Portal Users, both Customer Portal and Partner Portal Users, these columns will not display nor will the filter criteria used in the report be applied.