De optie concurrency=shell doet volgens mij niets meer in recente versies van Ubuntu (9.10 en nieuwer dacht ik), omdat nu upstart gebruikt wordt.
Edit: in 10.10 is "shell" niet eens meer een geldige optie:
# Specify method used to enable concurrent init.d scripts.
# Valid options are 'none', 'startpar' and 'makefile'. To enable
# the concurrent boot option, the init.d script order must allow for
# concurrency. This is not the case with the default boot sequence in
# Debian as of 2008-01-20. Before enabling concurrency, one need to
# check the sequence values of all boot scripts, and make sure only
# scripts that can be started in parallel have the same sequence
# number, and that a scripts dependencies have a earlier sequence
# number. See the insserv package for a away to reorder the boot
# automatically to allow this.
CONCURRENCY=none