One of the standard applications for in-game voice communication seems to be ventrilo http://www.ventrilo.com/ (at least for the cs:s masses). Despite a promise of a linux client, Flagship Industries will not be delivering any time soon (see http://www.petitiononline.com/vent4Lin/petition.html for more info).
Personally, I think the linux client is a lost cause (most people are making do with wine + ventrilo http://appdb.winehq.org/appview.php?iAppId=2169) or the teamspeak linux client. Both of these are proprietary binaries though...
insert Mumble here http://mumble.sourceforge.net/
Mumble has been progressing reasonably well of late http://mumble.svn.sourceforge.net/vi...y=rev&view=log (although not at the breakneck speed of PTS). Overlays work with most games, positional sound support has been implemented for battlefield 2 and WoW, it offers cross platform support and it is licensed under GPL...
Hardy seems to have picked up Mumble in its repositories too - Main and PPA (coinciding with Ubuntu defaulting to Pulse Audio). Perhaps it's time open source developers started integrating positional voice chat through mumble... openarena, nexuiz and warsow spring to my mind. I'm not sure how simple/difficult this integration would be but it would be the best way to further mumble development/adoption/awareness.
A brief overview of Mumble on Phoronix would help too
Has anyone else been thinking along these lines?