I just ran into this problem after an upgrade to 251-7.5, and Rene's problem assessment still holds true with this package version.
The debconf prompt indicates it is requesting a URI, but even when just pressing Enter to confirm the previous "ldapi://" URI, the libnss-ldap.conf file gets rewritten with a "host ldapi://" URI. This is a very sneaky bug - there is nothing in the logs indicating that there is a problem with libnss-ldap (other than the fact that all the LDAP users cannot be looked up anymore). If this can't be patched in time for etch, could a note about this be added to NEWS.Debian for this package?
