The Using Flash Remoting Update chapter of the ColdFusion Developers Guide documents how to integrate Flex and ColdFusion by using mx:RemoteObject tags to call ColdFusion Component (CFC) functions. It includes information on how to configure a Flex Builder project to compile an application that accesses ColdFusion, and describes how to specify and use CFCs in your Flex applications. Finally, it describes how to compile and run such applications.
The version of the chapter that was released with ColdFusion 8 was not updated correctly at that time, so it is not appropriate to use when integrating either Flex 2 or Flex 3 with ColdFusion 8. The updated chapter is (we believe) correct, and covers Flex Builder 2 and Flex Builder 3.