As of version 1.0 the specification does not clearly state, whether the static or the runtime type of an association should be considered when creating property paths in case of cascaded validations.
Let there be the following type:
When validating an instance of Foo (which has invalid elements in bar1/bar2) with the RI, the resulting property path node for bar1 would be iterable but not indexable ("bar1"), while the property path node for bar2 would be iterable and indexable ("bar2"). This is that way because the static type of bar1/bar2 (Collection vs. List) is considered by the RI when building the nodes, and not the runtime type (ArrayList in both cases).
According to a post  on the Hibernate Validator forum Apache's JSR 303 implementation follows the runtime type approach. This seems reasonable to me, too (as for instance the constraints to validate are also determined based on the runtime type of references).
The specification should make clear what the intended behavior is and provide a TCK test case.
 related forum post: https://forum.hibernate.org/viewtopic.php?f=9&t=1010626
 related discussion on hibernate-dev: http://lists.jboss.org/pipermail/hibernate-dev/2011-April/006357.html
FYI, Hibernate Validator already does this now already -