Beat Kiener has posted about reducing the SOAP message in an additional way beside the binary encoding.
When communicating in a .NET to .NET messaging scenario then optimizing the data contracts is an additional way to reduce the SOAP message size. This together with binary encoding will reduce the message size about 4 times. Especially looking to the Windows Phone 7 development this will help much when the traffic goes over a low bandwidth network like GSM or UMTS.