MySQL 5.5 is now in [extra]. This is a major version upgrade from the 5.1 editions previously in the repositories. Archive, Blackhole, and Federated engine support are no longer included in this package, since they are not heavily used.
After upgrading the MySQL package and restarting the database, you
will need to run mysql_upgrade to ensure your tables, views and
stored procedures are up to date and compatible with MySQL 5.5.
It is always good to do a backup of your databases before doing the update. You will also need to adjust my.cnf options in order to make it compatible with 5.5 release; more information is available here.