Using the etch DVD as local Debian mirror
When booting the Debian installer via PXE (see the relevant chapter in the Installation Guide for how to setup DHCP/TFTP and where to download the necessary files), the next logical step is to use a local Debian mirror in your network.
To use the etch DVD (multiarch or architecture-specific) as Debian mirror, you need to have a Webserver or FTP-Server on your network. Copy the files from the DVD to the disk (or loop-mount the ISO image if you like) somewhere the server can find them, and configure the server to provide for example http://yourServerNameOrIP/debian/ to access the installation medium ("/debian/" is just an example an can be any path you like).
Test downloading http://yourServerNameOrIP/debian/dists/etch/Release to see if the paths are set up correctly. One problem that remains is that the CD/DVD images of Debian installation media do not provide dists/etch/Release.gpg (which is the cryptographic signature of the Release file), so to get the installer to accept your mirror you need to add
to your kernel command line, either by entering by hand when starting the Debian installer or even better by adding it to the "APPEND"-line of pxelinux.cfg/default.
After that, the installer can be told that "yourServerNameOrIP" is the name of your local mirror and "/debian/" is the path on the mirror. It should then find anything necessary to complete the installation process. Of course Debian will also be able to use the mirror afterwards, the installer puts the necessary entries into /etc/apt/sources.list all by itself.