This topic with other methods has been covered previously here. However, it was considered as more of a "hack".

Here is a method that is currently supported by Debian itself, edit the /etc/init.d/rc:root:~# nano /etc/init.d/rc

Scroll down a page or so and you'll see a segment of script similar to this:

# Specify method used to enable concurrent init.d scripts. Valid
# options are 'none', 'shell' and 'startpar'
CONCURRENCY=none


Examining the script further down tells me that startpar is not fully supported, so I went with shell. A restart shows me that sure enough, the init.d scripts are being run concurrently. This did make quite a difference in my boottime, knocking nearly 20 seconds off. Of course, any disk read/write bottlenecks can still happen.

Enjoy.