In the Official Samba3 HowTo chapter 13. it says "Restoring the IDMAP Database Dump File The IDMAP dump file can be restored using the following command: net idmap restore <full_path_and_tdb_filename> < dumpfile.txt" According to the man pages and real behavior second line should read: "net idmap restore dumpfile.txt"
Fixed with b4a4e63 and added to all branches. Will be in the next releases. Thanks for reporting!