Because Tailspin is using a custom WCF service, they must use custom data transfer objects to exchange data between the mobile client application and the cloud-based service. Tailspin chose to use a Java Script Object Notation (JSON) format for moving the data over the wire because it produces a compact payload that reduces bandwidth requirements, is relatively easy to use, and will be usable on platforms other than the Windows Phone 7 platform.
Note: If, in the future, Tailspin moves to WCF Data Services, it will no longer require the custom data transfer objects because WCF Data Services use the OData protocol to move data over the wire.
Tailspin also considered compressing the data before transferring it over the network to reduce bandwidth utilization, but the developers at Tailspin decided that the additional CPU and battery usage on the phone that this would require outweighed the benefits in this particular case. You should evaluate this trade-off between the cost of bandwidth and battery consumption in your own application before you decide whether to compress data that you need to move over the network.