Starting from MySQL version 3.23, you can use LIMIT row_count to restrict the scope of the UPDATE.
A LIMIT clause works as follows:
* Before MySQL 4.0.13, LIMIT is a rows-affected restriction. The statement stops as soon as it has changed row_count rows that satisfy the WHERE clause.
* From 4.0.13 on, LIMIT is a rows-matched restriction. The statement stops as soon as it has found row_count rows that satisfy the WHERE clause, whether or not they actually were changed.
http://www.mysql.com/doc/en/UPDATE.html