This only works if you stick to a simple filesystem, not crypto, RAID, etc. Otherwise you'll need to have file system utilities (lvm2, mdadm, cryptsetup, zfsprogrs, etc) on the initrd to get your rootfs mounted.
It never crossed my mind that LVM and similar needs utilities to access the filesystem and to be honest, I thought initrd/initramfs contained only kernel modules, not executables and scripts... ^__^;
