(I've deleted obsoleted versions, as well as the postgresql-7.4 versions)

# repoquery --whatrequires libpq.so.3 | sort | uniq
cyrus-sasl-sql-0:2.1.19-5.EL4.i386
dovecot-0:0.99.11-4.EL4.i386
freeradius-postgresql-0:1.0.1-3.RHEL4.3.i386
libdbi-dbd-pgsql-0:0.6.5-10.RHEL4.1.i386
mod_auth_pgsql-0:2.0.1-7.1.i386
perl-DBD-Pg-0:1.31-6.i386
php-pgsql-0:4.3.9-3.22.3.i386
qt-PostgreSQL-1:3.3.3-10.RHEL4.i386