I stumbled upon this Technet Forum post. Using the SPListItem.UpdateOverwriteVersion method, you can overwrite Created, Created By, Modified and Modified By fields. Here is a code snippet from this MSDN article: 


      using (SPSite sps = new SPSite("http://server/"))

      {

          using (SPWeb spw = sps.OpenWeb())

          {

              SPList spSharedDocs = spw.Lists["Shared Documents"];

              SPListItem spli = spSharedDocs.Items[0];

              spli["Created"] = new DateTime(2007, 1, 1);

              spli["Author"] = "-1;#domain\\user";

              spli["Editor"] = "-1;#domain\\user";

              spli["Modified"] = new DateTime(2007, 2, 1);

              spli.UpdateOverwriteVersion();

          }

      }