Bug was reported on App Engine forums but I confirmed it with an rdbms plugin built from trunk.
Here's the app engine issue:
Basically, CascadeType.ALL on a ManyToOne causes a StackOverflow error when deleting the owning object if the OneToMany is also CascadeType.ALL. I'll attach a JPA project reproduces the issue. I haven't tried it with JDO. Let me know if you need any more info.