Short answer: first upgrade from unstable etch to stable etch, then from etch to lenny.
It looks like you didn't have an up-to-date etch installation, according to the discussion for bug #458452. Upgrades are supported from one stable release to the next, and between successive states of an unstable release (including its final stable state), but not always for more complex cases. To upgrade to a stable etch even though it's disappeared from Debian mirrors, get it from
aptitude is refusing to do anything until you repair the problem manually, you can run
dpkg --force-overwrite -i /var/cache/apt/archives/debianutils_2.30_i386.deb. However going via etch would be a better idea (and you wouldn't have this problem, because the
debianutils package from etch declares dependencues that cause
passwd to be upgraded first).
Note that when upgrading to a new release, you typically need
full-upgrade rather than