I just discovered what looks like another, related bug. A web service response containing arrays marshals correctly under Flex 2, but under Flex 3 Beta 2 returns 'null's in the 1st array encountered in the response.
I've opened yet another bug about this one: "Web service array marshalled correctly in Flex 2 not marshalled in Flex 3" https://bugs.adobe.com/jira/browse/SDK-13968 So now I'm between a rock and a hard place. I have a set of web services that use arrays in requests and in responses. Flex 2 doesn't handle response arrays. And Flex 3 is WORSE because it's not handling request OR response arrays correctly.