I want to put my Raspberry Pi upstairs and connect it to the network via Homeplug. On my main computer, I would like to play music via VLC using something like the MMC protocol. What do I need to do in order to get my raspbmc-enabled Raspberry Pi to automatically play the stream stream on startup, without needing an input device of any sort or even a display device after configuration?
It's simple, just write startup script for your distribution.
There are plenty guides on Internet for that.
Make sure that it starts after all required services are up.
Command may look like:
cvlc mmc://something > /path/to/log/file.log
cvlc would start vlc headless and standard output would log into your logfile so you can keep track of what is going on if something goes wrong there.