See related SO question.
If you do not specify a bind_ip then mongod relies on the hostname returned by gethostname(). If that hostname is not fully qualified (eg gethostname() returns "host" and not "host.example.net") then further lookups may fail, especially when trying to set up replica sets.

Net: for examples relying on localhost, make that explicit with bind_ip 127.0.0.1 or hostname localhost, otherwise use fully qualified domain names and note that the hostname of the system mongod is running on must be fully resolvable in DNS.