I am not sure if this is an actual bug in mythbuntu, but I have been using mythtv-trunk weekly builds (installing a new upgrade weekly), on 9.04 and this happened when I upgraded yesterday, even though I did not change any other settings.
Basically the symptom is that the /etc/init.d/mythbackend script hangs, causing both the "apt-get upgrade" operation and normal machine bootup to fail.
I found that the problem was that the "ARGS" environment variable, set in /etc/init.d/mythbackend, is overridden in /etc/default/mythbackend, and the original value "--daemon --logfile /var/log/mythtv/mythbackend.log --pidfile $RUNDIR/$NAME.pid", is replaced with "--logfile /var/log/mythtv/mythbackend.log". Since the "--daemon" argument is missing, mythbackend never returns control to the init script
Howdy, and thank you for not only creating this ticket, but proposing a fix. It is greatly appreciated!