After upgrading to e2fsprogs-1.17-1 on RH6.1 mkinitrd no longer works. It seems it is unable to create an ext2fs on a loopback device. The e2fsprogs package originally distributed with RH6.1 works OK though.
Should work with e2fsprogs-1.18-5 and mkinitrd-2.4.1-1.