While we at OsmocomTETRA have been looking only at implementing the TETRA protocols as they are (and doing a bit of sniffing on unencrypted networks), some researchers have recently published two ground-breaking papers on the (lack of) security in the APCO P25 radio system.
In case you haven't heard about APCO P25: It is a digital mobile radio system mainly used by Police in non-EU English speaking countries like the US, Australia and New Zealand.
So apparently P25 uses either single-DES or a proprietary cipher with only 40 bit key-length. No, I'm not joking. Seems like it was developed by people who have not the slightest clue about communications security at all.
And guess what they used to receive and transmit P25 waveforms? Of course the USRP and gnuradio. This once again proves how invaluable those tools are, not just for the FOSS community, but also for the communications research community.