While trying to resolve a ports issue on a staging server, I happened to run an nmap on our production server, and found that port 800 was open:
800/tcp open mdbs_daemon
I have no idea what mdbs_daemon is - I have googled the port number and the daemon, but can only find a discussion hinting that it is something to do with NFS, but I'm pretty sure I've not installed anything related to that on my Ubuntu 10.04 production server.
I'm just not sure how concerned I should be at this unexpected port.