I have 2 locations that I am trying to setup a webcam video conference
system for.   One has only Macs (a laptop running OSX and a desktop
running OS9.2), and the other has a Windows XP computer.   Each has a
webcam and microphone hooked up (the Mac location has a webcam for
each Mac).

I need some kind of webcam software that has both Mac and Windows
versions and that can deal with computers behind a NAT router.

The NAT router part I think will be the hardest part.  The macs are
behind an Acatel router that has no DMZ zone or Port Forwarding
features.  They are both on 192 addresses.  The PC is behind a Linksys
router that does have port forwarding.  It would be great if there was
software that was “NAT-aware” like MSN Messenger (they sometimes call
it.NET Messenger, or sometimes Windows Messenger) is.

I tried MSN Messenger because I heard that can get around NAT, but the
Mac ver does not support video, only text chatting.  I also tried
iNetcam’s iVista, but it was a paid to setup the port forwarding on
the Linksys router to make it work, and it is PC only.

You can use Macromedia's Flash Communication Server. The personal
edition costs 499 USD, but there is a 30 day trial you can download:

http://www.macromedia.com/software/flashcom/

The media is delivered using the Flash 6 plugin, the standard whenever
you want to target a cross-platform audience. The flash 6 plugin also
contains the software necessary to access a webcam and microphone.

Would this suffice as an answer?

All the best
skorba-ga

Does it get around the NAT issue?

I guess it would work in a worst case scenario situation, but I really
need something that doesn't require setting up and co-locating a
server.  Also, the $500 is very very steep for just doing webcam
chatting.