D Star isn't proprietary, it's an open standard developed by amateurs in Japan to test the applicability of digital transmission to ham radio. Icom has decided to commercialize it, but with mixed results so far. Since most hams still have analog gear, I don't expect to see a lot of either P25 or D Star in the future. You won't see much, if any, ham gear sold with P25 due to the cost of licensing the IMBE VoCoder. All of which means that you will have to buy commercial gear to convert for use on the ham bands if you want P25.
Unlike FM, SSB, or AM, not all digital modes are compatible. APCO P-25 is open standard, available to anyone who wants to pay the licensing fee to the developer. D Star is open source, which means that anyone can have the source code, which you most decidedly CAN NOT do for P25.