Whenever VOIP and GPL'ed software are mentioned I always think of asterisk. It's safe to say that asterisk has a history that is as interesting and as long as that of GNU Radio :-). (Another buzzword to keep an eye on in this area BTW is "ROIP").
Anyway asterisk (in addition to being the GPL swiss army knife for VOIP) has very highly articulated two-way radio and radio repeater module which is known as app_rpt - their project page ("The fusion of 2-way radio systems and VOIP") is at
http://ohnosec.org/drupal/.
While the focus of app_rpt historically has been on analog (NBFM) radios transmitting and receiving on discrete frequencies, our project (op25) offers a plugin channel driver to extend a bridge between the asterisk/app_rpt/VOIP world and GNU Radio. When
using the USRP hardware it aims to support a fully generic, multi-channel, FDX two-way radio infrastructure with a specific focus on P25. We're always looking for experimenters and amateurs who are interested in these technologies... Project pages are
http://op25.osmocom.org/trac/wiki
http://op25.osmocom.org/trac/wiki.png/wiki/RepeaterPage
Lots of work still to do (of course), including all the new goodies Jonathan Corgan has been mentioning that are in the GR pipeline for dealing with packet-oriented datacomm protocols...
Best Regards
Max