mythbuntu's casper hooks appear to be in the mythbuntu-live-autostart package.
$ dpkg -c ~/ubuntu/pool/universe/m/mythbuntu-live-autostart/mythbuntu-live-autostart_0.57_all.deb |grep casper drwxr-xr-x root/root 0 2011-08-29 07:43 ./usr/share/initramfs-tools/scripts/casper-bottom/ -rwxr-xr-x root/root 814 2011-02-25 06:33 ./usr/share/initramfs-tools/scripts/casper-bottom/46disable_services -rwxr-xr-x root/root 1484 2010-09-12 15:25 ./usr/share/initramfs-tools/scripts/casper-bottom/47fixup_icons -rwxr-xr-x root/root 588 2010-08-25 17:46 ./usr/share/initramfs-tools/scripts/casper-bottom/48maybe_ubiquity $
mythbuntu's casper hooks appear to be in the mythbuntu- live-autostart package.
$ dpkg -c ~/ubuntu/ pool/universe/ m/mythbuntu- live-autostart/ mythbuntu- live-autostart_ 0.57_all. deb |grep casper initramfs- tools/scripts/ casper- bottom/ initramfs- tools/scripts/ casper- bottom/ 46disable_ services initramfs- tools/scripts/ casper- bottom/ 47fixup_ icons initramfs- tools/scripts/ casper- bottom/ 48maybe_ ubiquity
drwxr-xr-x root/root 0 2011-08-29 07:43 ./usr/share/
-rwxr-xr-x root/root 814 2011-02-25 06:33 ./usr/share/
-rwxr-xr-x root/root 1484 2010-09-12 15:25 ./usr/share/
-rwxr-xr-x root/root 588 2010-08-25 17:46 ./usr/share/
$