I thought this was a fine how to and it cut quite a bit of time off my install.  I did a few things differently, but it was handy to be able to just "walk through" the rest of it without thinking about it.

A couple of notes I'd throw in:

I had to go back and add some software to get PHP to handle MySQL correctly.  In step 9, you could add the libmysql++-dev package to avoid the problem.  (I believe I had to add this package because I took a slight detour loading PHP and MySQL.)  It won't hurt to put this package in your apt-get statement in either case.

In step 11, it is probably a good idea to use nslookup to check your DNS is working right.  I have a bit of "wonkiness" going on with my network and use a rather odd DNS setup.  If DNS isn't working right, ISPConfig won't load.  

In my case, I was loading from scratch, and got a DHCP address the first time through.  Everything worked for the install.  When I got to where I had to set up my "real" IP address, I didn't quite get it right.  It pays to stop at this point and verify your DNS is right.

This is a whole lot handier than starting from scratch with a "roll your own" distribution like Slackware.  Very nice.