well an update if you really want to know...
the init system was replaced by upstart but init still starts and manages processes (like an upstart alias).
upstart/init is in a transitional stage at the moment from what i can tell.
a lot of the init file structure is still there so as to not scare everyone with a sudden change but remember all those rc?.d/* are just a bunch of directories and links to init.d/*.
Some facts (to scare you).
Upstart is event based. eg system starts, Ctl-alt-del pressed, some process stopped, flag set to...
event files are located in /etc/event.d/
upstart does not have or know about runlevels. ie there are no run levels any more.
Ubuntu/Mint OS's now run in "runlevel" 2. not runlevel 5 as we are all use to. but remember there are no run levels any more so who cares what it is called. this also means you could delete rc3.d, rc4.d and rc5.d and not harm your system. well it has not hurt mine yet.
run levels, and so /etc/rc?.d could be replaced with events:
runlevel 0 = rc0.d = shutdown system
runlevel 1 = rc1.d = system maintenance mode
runlevel 2 = rc2.d = run system
runlevel 6 = rc6.d = reboot system
maybe they should be called major events?
there is a new event level S or Startup that runs all sorts of stuff at boot and before any other runlevel. found in /etc/rsS.d/.
well more later