I recently wrote an article on using JSON on mobile Java applications. From the article:

Choosing the right data interchange format is an important design decision when building any network-aware software. This is especially true when designing mobile and embedded applications, where attributes such as lightweight and efficient are important characteristics to consider. Such characteristics are important because they translate to lower computation requirements and use of power, potentially better performance, and lower costs of operation.

See Using JavaScript Object Notation (JSON) in Java ME for Data Interchange (Sun Developer Mobility website).