GNU radio (http://www.gnuradio.org
) has a GMSK module available. However, D-star uses a vocoder chip called the AMBE-2020 (http://www.dvsinc.com
) that digital voice systems has clearly said is not going to be made available in a software format, nor will it license the technology to other companies. That alone is reason enough to cross D* off my list. The other major downer is that the data transport is ATM, which is only really useful for voice (and interfacing with the legacy switched phone service) without having a lot of nearly empty packets and extra headers.
The other thing is that we've only seen Icom radios and repeaters.
Personally, I like the idea of using GMSK modulation with Ethernet framing. Easy to implement for homebrewers, standards based (using open source vocoders), and more ham-friendly than spread spectrum. Once I get a few bucks saved up, I'm going to start messing around with it.