Prototypes and proof of concept demonstrations are powerful tools for validating your concept prior to committing to a full-blown implementation of your handheld application.
Prototypes in the mobile world typically center around the design of the application on the PDA so that you can assess its user interface, navigation and ease of use.
Invariably once you begin to see your application come to life, it will spark ideas about the application's design and overall flow and the end result is a handheld application that more uniquely fits your needs.
Proof of Concept Demonstrations often involve testing the ability of the handheld to perform well with an external device, technology or application so that you can assess the feasibility of your concept prior to investing in the final implementation.
Immensely useful in "selling" a full implementation to investors or to your internal user group, prototypes and proof of concept demonstrations can be a cost-effective way to validate your mobile solution.