If you need to get a return value as soon as the Invoke is processed (included in a block), your best bet is to use some events (for the moment I guess).
In your chaincode, just setup the event with:
func (stub *ChaincodeStub) SetEvent(name string, payload byte) error
You may be able to listen for events in your application using the SDK or protobuf messages directly. I'm doing it like this on the developer preview; but it seems that the standard way to retrieve Invoke result is to poll the blockchain via Queries.
There is a related GitHub issue here.