in Planet Debian ~ read.

reinstalling all packagese

for a in `cat /var/lib/dpkg/status | grep -B 1 installed | \
	grep -v installed | perl -ne 'if(/:s+(.*)/) {print qq($1n);}'`; \
do apt-get --reinstall install $a; done

or

# COLUMNS=200 dpkg -l | grep ^ii | awk '{print $2}' | \
xargs apt-get -y --reinstall install