I'm currently researching whether BlazeDS for accessing Java Objects from a Flex client makes sense.
BlazeDS is supposed to be much more efficient than other alternatives such as JSON or SOAP.

Unfortunately it seems that Adobe's open source BlazeDS does not allow one to use HTTP's caching infrastructure. This pretty much renders the more efficient binary AMF protocol pretty much useless for scaling to many users. Fewer requests are almost always better than smaller requests.
Live Cycle Data Services Adobe's commercial variant of BlazeDS has sophisticated offline support. This might help to get things cached, but first it's commercial and second it might be to complicated, if you don't intend to develop full offline capabilites.