While playing around with report rendering in 2005, I noticed RGDI (for example, in CTP15 use Report Manager to create a subscription using the File Share delivery extension and you'll see "Remote GDI+ file" in the list of render formats).
Anyway, the RGDI renderer is used by the new Windows Form Report Viewer control, and really is meant for client-side scenarios. I asked around and found that even though you can render a report as RGDI on the server and save it to a physical file, there's not too much you can do after all. We don't give you an easy way to hook that file back INTO the Winform report viewer.
So now you know what RGDI is and what you can/can't use if for :)