Right now, I'm doing most of my testing using a qemu/kvm guest and
generating the initramfs on another box but the support is all present
to build for the "running" machine.  For the former, you can boot the guest
using qemu's -kernel and -initrd options.  Currently supported rootfs
types are regular partitions, root-on-lvm and root-on-lvm-on-encrypted-pv.

dracut exists and will build an image.  It is command-line equivalent
to most mkinitrd implementations and should be pretty straight-forward
to use.  

To use, just run dracut with an output file name and, optionally, a
kernel version (it defaults to using the current).   The appropriate
modules will be copied over and things should be good to go.  If you'd
like to customize the list of modules copied in, edit /etc/dracut.conf
and set
  modules="foo bar baz"


Requirements:
* udev