Important! If you are running a service that relies on forward/reverse lookup checks, don't do this! An example would be, if you are running an smtp server which only accepts email from a host with a valid ptr record.
The Name Service Cache Daemon (nscd) has a default behavior that does not allow applications to validate DNS "PTR" records against "A" records. In particular, nscd caches a request for a "PTR" record, and when a request comes later for the "A" record, nscd simply divulges the information from the cached "PTR" record, instead of querying the authoritative DNS for the "A" record.
As far as I know, this is still the case. It is terribly unimportant for an average home user.
Netscape, Konqueror, Opera and Mozilla all have their own built in resolvers. They utilize the DNS resolver libraries but don't query the nscd db.