I think it should be possible to change the parent of a directory to null:

Sample Scenario:
Filebrowser that manages a directory structure (the directories are secured by acegi). Now i want to move a nested directory to Root-Level. The parent-property of the corresponding ACL has to be changed to NULL.

Solution:
Simply change:

    public void setParent(Acl newParent) {
        aclAuthorizationStrategy.securityCheck(this, AclAuthorizationStrategy.CHANGE_GENERAL);
        Assert.notNull(newParent, "New Parent required");
        Assert.isTrue(!newParent.equals(this), "Cannot be the parent of yourself");
        this.parentAcl = newParent;
    }

to

    public void setParent(Acl newParent) {
        aclAuthorizationStrategy.securityCheck(this, AclAuthorizationStrategy.CHANGE_GENERAL);
        Assert.isTrue(newParent == null || !newParent.equals(this), "Cannot be the parent of yourself");
        this.parentAcl = newParent;
    }