What's your actual problem? My (truly uneducated, but user-gained) understanding of this is that the port opens when a request is made. When I nmap as you did:
mythslave ~ # nmap -P0 localhost
Starting Nmap 4.11 ( http://www.insecure.org/nmap/ ) at 2006-10-13 02:17 MDT
Interesting ports on localhost (127.0.0.1):
Not shown: 1673 closed ports
PORT STATE SERVICE
22/tcp open ssh
111/tcp open rpcbind
970/tcp open unknown
2049/tcp open nfs
6543/tcp open mythtv
6544/tcp open mythtv
Nmap finished: 1 IP address (1 host up) scanned in 0.124 seconds
My TFTP port doesn't show up either, but I have a working diskless setup.
So, what's the actual problem you are having?