You could do things this way, but it's not clear to me why you wouldn't just use Rails' Web services.
As I understand it, AWS isn't going to disappear - it just may turn into an add-on rather than remaining part of the core Rails package. For those (apparently few) of us who use it, AWS is a really powerful, elegant way of working; REST makes it simpler to create simple XML documents, but the effort involved in turning those documents into SOAP or XML-RPC is pretty painful if you code it yourself.
Regards
Dave M.