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 most preseeding support has been added after rc1 of the installer.
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