Does MySQL 5.0 include a disk-based MySQL Cluster?

Not yet - essential infrastructure for this feature is currently under development, but the full feature is scheduled on the MySQL roadmap as a "rolling feature". This means that it is not a flagship feature, but will be implemented, development time permitting. Specific customer demand may change this scheduling. See also http://dev.mysql.com/doc/mysql/en/mysql-5-1-cluster-roadmap.html