MySQL 5 lets you define how the database engine reacts to bad data via the sql_mode variable.

By default, bad data is converted to “good” data (0-filled, or turned into a string) and inserted into the database. Adjusting sql_mode to strict_all_tables, causes the engine to rejected the data and to stop processing.

The recommendation for new installations of MySQL 5 is to make server-enforced data integrity the default (this involves modifying my.cnf file).

Originating Source: MySQL AB :: Guaranteeing Data Integrity with MySQL 5.0
MySQL Reference: http://dev.mysql.com/doc/refman/5.0/en/server-sql-mode.html