The man page for nmblookup says that "nmblookup -M -" searches for a master
browser for a network by searching for the machine with the "__MSBROWSE__" name.
This works with Samba 2.2.7a, but not with 3.0.0. With 3.0.0, nmblookup returns
the usage summary information, as if the command had been mistyped.
In 3.0 you need to escape the - sign on the command line:
nmblookup -M -- -