Here are the answers to a couple of questions I've been asked about my XBMC setup. These were already answered in the comments, but I thought I'd post them here for anyone who missed them.

1) Why do .SRT subtitles appear in the middle of the screen and how do I fix it?

It seems that XBMC doesn’t update the screen dimensions properly when the window size changes, so when it does it’s subtitle calculations, it’s actually positioning them for the size of the XBMC window when it originally started up. To fix it, you need to set the screen size in XBMC to the same dimensions as the desktop that is displaying on the monitor. In my case, I’ve got a 1080i TV showing a 1920×1080 desktop, so I’ve set the screen in XBMC to “1080i 16:9″. This particularly applies when you are using wmctrl to force XBMC to release the mouse in fullscreen.

2) How do I make XBMC automatically start after booting?

To get XBMC to auto start, I created a shell script which is called from “Startup Applications” under System->Preferences. This script will launch XBMC a few seconds after the user has logged in. The delay gives the PC a little time to settle down after starting.

#!/usr/bin/env bash
sleep 5 &&
xmodmap -display :0 -e “clear control”
xmodmap -display :0 -e “clear shift”
xmodmap -display :0 -e “clear mod1″
exec xbmc
exit

This script also disables the control keys so that a Jaycar/Digitech 4889 remote can be used, and is a suitable quick hack for setting up a dedicated media player box. There are better ways of doing it (ie. hide the Ubuntu desktop while starting up), but this is good enough for my needs.