After a bit of searching, it seems a possible solution is to reinstall my video drivers (nVidia GT210), since that is the common solution when booting to the login prompt. I did this ("sudo apt-get purge nvidia*", "sudo apt-get install nvidia-current"), but I am still booting to the tty1 login prompt. Audio/Video playback is still fine, and XBMC performs as before, it's just not autostarting, despite the GRUB kernel line reading as follows:
kernel /boot/vmlinuz-2.6.35-22-generic root=UUID=[UUID removed] ro xbmc=autostart,nodiskmount loglevel=0 quiet splash
BTW, it looks like the only GRUB-related file that was modified in the update was /boot/grub/grubenv, but it appears identical to the pre-update version.
Any help would be much appreciated.
P.S. I've updated XBMC fine before with the ro switch included in the above GRUB line, and files on the boot drive were updated successfully this time with it included, but could that be related to my issue?