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!