Since a few days the last version of squeeze’s debian installer (which runs sid I think) is able to install on an ext4 filesystem. Which means that the ext4 filesystem could now easily be used as root filesystem.
When the installation is completed a problem might appear at the first boot. The kernel will simply panic complaining that he cannot mount the root filesystem. Actually the squeeze’s default kernel is currently 2.6.26 and this version does not fully support ext4.
The quick fix is to use the rescue part of the debian installer which will chroot you in your root partition and let you temporarily change your apt’s sources from squeeze to unstable so that you can install the last unstable kernel (>= 2.6.28) wich is ext4 capable.
If you try to install with the stable debian installer or any GNU/Linux distribution which is not ext4 capable here is a trick to use ext4 as your root filesystem.
First install your system with an ext3 filesystem and let /dev/sda1 be that partition. Then install grub2 and make sure it works perfectly. Then boot on a live system which is ext4 capable, backup all files so you can create an ext4 filesystem on /dev/sda1, put your backup back in it. Then change fstab in /dev/sda1 and replace ext3 with ext4 for the root filesystem. Check that grub.cfg in /dev/sda1 doesn’t use UUID to pass the root parameter to the kernel (actually they’ll change since you made a new filesystem) and use root=/dev/sda1 instead. Now reboot and your system should be running ext4 on the root filesystem.