Just to put another solution out there. I don't like the idea of waiting 30 seconds for x to start. It's hackish and assumes that X will start in 30 seconds. I use gdm3 and have daemon.conf set to automatically log in the bitcoin user. Then in /home/bitcoin/.gnomerc, you can have all your miner start code, not having to care about $DISPLAY or whether X has launched properly. Further, this doesn't require creating a custom init.d script.

I think this could be adjusted further by taking out gnome entirely and having a line like:
@reboot startx
in a user crontab then using a .xinitrc to launch things.