Most of the questions asked by DebianInstaller can be preseeded by setting the answers in the debconf database. This can be used to do automatic installs. This page gathers our collective knowledge on the topic, so please add any tips you have.
There are currently two main mechanisms to preseed debconf values. Values can be set on the installer's boot line, for example this is useful for testing the unstable version of the installer in a netboot install:
The preseed udebs can be used to load a file containing values to preseed, either from the network or local media. The installation manual explains how to set this up: http://d-i.alioth.debian.org/manual/en.i386/ch04s07.html#automatic-install and includes a complete example. Note that you can check preseed-files for errors with "debconf-set-selections -c $file".
Also note that most preseeding support has been added after rc1 of the installer.
If you need the preseeding feature for a fully automated installation in a network with dhcp server be careful to not exceed the maximum of eight parameters for the kernel to boot. This could be a slight problem if you want to use a different language, keymap and country. Another fact: if you want to use dhcp to set the hostname and domain automatically, you need to pass "debconf/priority=critical" to the kernel at boot time and set "d-i netcfg/get_hostname" and "d-i netcfg/get_domain" to "string unassigned-hostname" and "string unassigned-domain".
Post here any links you have to example preseed files. Note that using any of these files directly is not wise, as a malicious person could probably come up with values for a preseed file that makes d-i misbehave. Also, the files are downloaded over http, so are vulnerable to man in the middle spoof attacks. The best way to use any preseed file is to copy it to your own local web server or media, and look it over before using it.
Joey Hess's modular/hierarchical preseed configuration, supporting installs via usb keychain or netboot with varying configurations for different machine types. http://kitenet.net/~joey/d-i/preseed
Christian Perrier's page documenting automated d-i installs in vmware, using netboot. http://people.debian.org/~bubulle/d-i/vmware-fai.html
Simon Kirkby's fully automatic basic install. !Warning! will format machine without asking. http://interthingy.com/digby
Holger Levsen's d-i examples showing a way to preserve partitions and ssh-host-keys: http://layer-acht.org/d-i/
Using network-console and preseeding is described on an extra page, ?DebianInstallerNetworkConsole
Hard won knowledge I
There are several ways to overcome the limitation of 255/225(?) characters on the kernel command line, such as using local preseed files. This gem http://lists.debian.org/debian-boot/2005/03/msg01061.html states that a local preseed.cfg will automatically be used when placed in the root directory of the initrd.
Hard won knowledge II
add "languagechooser/language-name=English" to the append line of your boot loader, it can't be preseeded any other way. (Thanks, Christian Perrier)
Hard won knowledge III
To make partman continue automatically, be sure to have the partman/confirm set to true in your preseed. (Beware, this fsck's up your drive real good!) preseed.cfg:
d-i partman/confirm boolean true
(Thanks, Christian Perrier)
Hard won knowledge IV
As Joey said, do not use /preseed.cfg in your initrd.gz (I should have listened). I tried it and failed, it couldn't get the local mirror. Using the exact same preseed file with preseed/url=... worked like a charm. Now I got to find the way to make it reboot automatically at the end of the installation process