I think maybe ventrillo has a mac client... However the free one is only 8 people max and you have to pay for a server if you want more. Don't know more than that really as my guild uses teamspeak since we got a guildy who works for some local ISP up where he lives and was able to put up a server... (On a 1gig pipe to the internet also, shame due to routing its 100megs lol)
____________________________
In a distant land, sink into the darkness...