In order to pass strong-typed objects from Flex to WCF (or Java) is to use remoting. WebORB and Fluorine are able to provide remoting to Flex using .Net. When a Flex application is using remoting the data is in a binary format (AMF). Having this provides faster downloading and (de)serializing of data.
Below are articles on this subject:
// here is an application that illustrates the power of AMF compared to plain-text formats.
I hope that helps, let us know if you are using SOAP, REST or remoting.