I have the following simple test case for this problem:

1) Create NodeA
2) Create NodeB
3) Create NodeB/Rel with reference property pointing to Node A
4) session.save()
5) Delete NodeB (should delete all its children + refs?)
6) session.save()
7) Delete NodeA (should be ok to do if B was deleted)
8) session.save() ==> results in Referential Integrity Error

It seems that the reference from the property in NodeB/Rel is not deleted when NodeB is deleted.