Please note that the method you recommend for activating libpam-cracklib only
applies to lenny and earlier; it's obsolete for the upcoming squeeze release
(and in Ubuntu 8.10 and later), where libpam-cracklib is integrated with
pam-auth-update
and will be autoconfigured. Running your sed command on a squeeze system will
break pam-auth-update integration, and cause the admin headaches going
forward if they ever have occasion to use anything other than pam_unix for
authentication.
So for a squeeze system, this should be:
apt-get install libpam-cracklib
... and you're done!
Actually, the package defaults in squeeze are stronger than what you've
listed: libpam-cracklib will set minlen=8 by default (instead of the
uselessly-weak minlen=6), and coming soon, pam_unix should default to
sha512 instead of md5 for password hashing.