missing mkinitrd

[Carl Alexander]

On an edgy system (recently upgraded from dapper), running
aptitude install xen-image-xen0-2.6.17-6-server-xen0 finishes with 

"Setting up xen-image-xen0-2.6.17-6-server-xen0 (2.6.17-6) ...
Please manually create an initrd image"

Googling on "initrd image" suggests that this is just a matter of
mkinitrd /boot/initrd-<kernel>.img <kernel>.  Which left me
wondering "if it's that easy, why didn't the installer just do
it?" Which caused me to look for the mkinitrd man page, which in
turn caused me to realize I don't have a mkinitrd binary. And
apt-cache search mkinitrd gave output that didn't seem all that
useful: 
bootcd-mkinitrd - bootcd extension to create an
initrd-image usable for bootcd yaird - Yet Another mkInitRD

Does anyone know what's actually necessary in order to be able
to boot from the xen0-2.6.17-6-server-xen0 kernel?

Thanks in advance!