Great work, l.capriotti!
I did not know ubuntu splash screens with the XBMC logo and xbmc-live scripts were in the APT repositories
I already was half way getting my normal Ubuntu Jaunty system to start XBMC at system boot without trashing my existing config including GDM. I modified the runXBMC script from the live CD to start X on a :1 display. But the downside was, you first see GDM booting and only after that, the screen switches to the new X session with XBMC.
The only thing that startled me (maybe this should be made more clear before installing xbmc-live) is that GDM and the ubuntu-desktop meta package get removed :o. But OK, in my case, I only use an old CRT TV so desktopping on it was a PITA anyway. But again, I could imagine that some people might only want to remove GDM from the startup proces but be able to start it manually
This brings me to another point:
Shutting down the system from XBMC works basically, but what worries me (it does not look nice) is that XBMC seems to quit and immediately starting up again (using the runXBMC/.xsession loop), to be killed by the system shutdown proces yet again
A last note: Is it wise to have XBMC start at position 20 in the boot process? In jaunty, this process seems to be somewhat paralellized and at a first glance, I don't see things started after S20 that XBMC relies on heavily. But in my own situation, I had start up scripts for my IRTRANS displayl and ir reservers on position S97 and S98, so it would be good if readers of your guide would be aware of this.