The only part that's closed is the voice codec (AMBE). But if you just want to transmit or decode data packets, you can definitely build your own hardware. There's even a couple SDR modules for it:
http://www.flexradio.com/amateur-products/flex-6000-signatur...
http://www.rtl-sdr.com/listening-d-star-digital-voice-dsd-1-...