I think there may be a bug in the way web service
serialization works in the latest release of flex 2.0. In beta3 web
service element of type array(elements whose maxOccurs="unbounded")
were deserialized by flex in to an object of type Array. However in
the new release web service arrays are now deserialized in to an
object of type ArrayCollecton. The strange behavior comes when Flex
tries to serialize an ArrayCollection. An error is thrown.
In other words
it appears that flex deserializes arrays (elements whose
maxOccurs="unbounded") into ArrayCollection but can not serialize
an ArrayCollection in to an array. It can however handle
deserializing an Array.