Garmin chirp uses the ANT+ protocol and remains compatible with Garmin GPSMAP (62s, 62st, 78s, 78sc), Oregon (300, 400t, 4001, 400c, 450, 450t, 550, 550t, and x50) and the Dakota 20.
The chirp, however, is not compatible with other Garmin GPS handsets such as Dakota 10, Oregon 200, GPSMAP 62, GPSMAP 78, all GPSMAP 60 and 76 models, and all eTex and nuvi models, that do not use the ANT+ technology, besides non Garmin GPS devices. Surprisingly, even Garmin Colorado, which works in the ANT protocol does not seem to find support. This is a major drawback of the chirp, which can limit its widespread application.
Older Garmin geocaching units compatible with the chirp, will need to update the GPS software before using the chirp. This process usually goes on smoothly with little or no bugs, but may face some obstacles depending on the type of handset and the settings.
The chirp operates via ANT+ through the newly created ANT+ Geocache Node Device Profile, which is available and accessible to hundreds of companies. So theoretically, any ANT+ vendor should be able to develop an iPhone app or Android app that could operate with chirp.