Ext4 support is only officially available since debian squeeze (6.9) release.

Instructions on how to setup a new debian using the official installer were found here. The installer method was prefered over the lenny tarball installation to get a better control over the installation. Only difference to the method proposed in the article was the usage of the squeeze installer instead of the lenny installer in order to get the ext4 support.

Nice to see that the kirkwood platform is still supported by the debian project !