How to Change the “Document Created By” field in a SharePoint Documents Library with code?

Hello,

I have a code that changes the username in various SharePoint lists, mostly by the "Author" column. It all works fine on normal lists, but it doesn't seem to work on the "Shared Documents" list which is a document library.

Whether I change the username in "Created By" or "Document Created By" (both columns exists in a documents list), the change doesn't seem to take effect. The item.Update command doesn't throw any exception, but it clearly doesn't update the field(s).

What can I do if I want to change this field through code?

(before you ask - I do set AllowUnsafeUpdates to true) 

Thanks