Using Ubuntu 9.10 Linux and the information and software in the following links I've managed to get some rudimentary P25 decoding using a discriminator output.
Original RR thread: http://forums.radioreference.com/tru...er-mbelib.html
Newest RR thread: http://forums.radioreference.com/tru...-released.html
RR Wiki Page: http://wiki.radioreference.com/index.php/DSD
In addition to P25 decoding the software apparently works with some success with other digital voice modes. See threads for details.
I am getting choppy but intelligible decodes with it. As mentioned it does require Linux but reportedly a port to Windows is in progress by someone. If you run Linux and have P25 signals in your listening range I'd encourage you to give it a try. Post #30 in the original thread offers a good outline for compiling the software and running it. I followed it verbatim and it worked for me, and my knowledge of Linux is "novice" to say the least.