I have same problem and fixed it.
I had the same problem. smbd would start, but after reloading the server it would always give me an error stating that the nmbd pid was not valid.
I then restarted samba (and it said it was starting smbd and nmbd) but after checking the processes (ps -A | grep nmbd) I found it was not running.
I checked the /var/log/samba/log.nmbd log file and found that it could not get the IP address I had set in the interfaces= section of the smb.conf.
I had it originally using eth1 through a switch (for testing) and then changed it back to using eth0 through my router. Unfortunately I did not notice that I gave the address of the router instead of the IP of the computer. So the router wasn't going to let that IP address get used.
I changed the interfaces= part of smb.conf to match the IP address of my computer and the interface (eth0). All works fine now.