I'm reporting this for the version of Samba that I'm running (Debian stable), but I don't think the issue is unique to that version.

The code of the nmbd program calls select() in the listen_for_packets() function
in nmbd/nmb_packets.c. If this returns successfully, it then calls read_packet()
to read from a socket. The read_packet() function in libsmb/nmblib.c calls
read_udp_socket() in lib/util_sock.c, which calls sys_recvfrom() which is a 
wrapper around the recvfrom() system call.

The problem is that the recvfrom() is a blocking call, so if there is no
datagram waiting to be read, the program ends up waiting forever. I haven't
puzzled out yet why this is happening on my machine, but the man page for
select() explicitly warns that a read might block when select() had reported the
handle as ready for reading, so read following a select() should always be
non-blocking. So I think the blocking read is a bug in the Samba code, at least in the Linux version.