I posted in my previous entry (Timestamp based migrations in Rails 2.1) about the new UTC timestamp based migrations in Rails 2.1. After working with them a little more, I found how to apply or remove individual migrations. Simply run rake db:migrate:up VERSION=YYYYMMDDHHMMSS to apply a single specific migration, or rake db:migrate:down VERSION=YYYYMMDDHHMMSS to remove a specific migration.
However, I found a bug with db:migrate:down as noted at http://rails.lighthouseapp.com/projects/8994/tickets/369-db-migrate-down-does-not-remove-migration-version-from-schema_migrations-table. When you run this, the self.down function of the migration is called and the database is changed. However, the entry in schema_migrations for this migration is not removed. So if you run rake db:migrate later, this migration will not be applied because Rails thinks that the migration has already been applied. My work around for this is to manually delete the row in schema_migrations after I run db:migrate:down.