I'd be very interested in this, I've been trying to get Asterisk working for my LAN (and through an external modem), but have only managed to get as far as it taking a message for me.
I hope you're going to publish docs about how your tests go, and what software you look at :o)
I've just installed Skype (www.skype.org), an IM looking VoIP app, looks like it could be pretty good, and also does conferences. Not available for mac though, which is a shame since a few of my mates have macs, and I'm also not sure whether you can use it as a standard VoIP program, or only on their network.
Duncan Sample Antimatters
T: 07050 801455 F: 07050 801895 E: ds@antimatters.co.uk W: www.antimatters.co.uk