If I replace a MySQL 5.1 server with a MariaDB Server (Maria & XtraDB storages) instead of MySQL (MyISAM & InnoDB), will most of MySQL client software (incl. applications made with PHP 5.2 and Java SE 1.6) ...
http://kb.askmonty.org/v/mariadb-versus-mysql
All MySQL connectors (PHP, Perl, Python, Java, MyODBC, Ruby, MySQL C connector etc) works unchanged with MariaDB.