It is in

As of 2010-03-06, the performance schema is merged into MySQL version 5.5.3-m3, in mysql-trunk.

The documentation for 5.5 has also been updated, and contains a new MySQL Performance Schema chapter.

Disclaimer

Please note that MySQL 5.5 is not a GA product. The performance schema feature may still change at any time, for any reason, and without notice. Customers should not make purchasing decisions based on the availability of the performance schema.

Marc Alff, Oracle.