many 'unauthenticated user', 'connecting host', in show full processlist (Mysql 5.1.5)
Setting skip_name_resolve disables the MySQL server's ability to perform DNS lookups. DNS servers can be unavailable and for a production server it is usually better to specify and ip and not a hostname as the 'host' specifier of a username. However users must then be created as 'user'@’ip’ and not as 'user'@'host' (except for ‘localhost’ what is accepted with this setting ON)

Do not resolve host names when checking client connections. Use only IP addresses. If you use this option, all Host column values in the grant tables must be IP addresses or localhost

http://dev.mysql.com/doc/refman/5.0/en/host-cache.html