Most computers can be set up to run multiple networks.
The most simple is to use an Airport card to connect to one network and Ethernet to another.
What can be a problem for iChat is when the second network also has a connection to the Internet.
Say you have your computer Ethernetted to your Modem for your Internet Connection. Then in another computer you set Internet Sharing from it's Ethernet over it's Airport card to the first computer by it's Ethernet card.
If we say the first modem gives your computer an IP of 192.168.1.1 and the second computer gives you one of 169.245.1.2 - your computer is sitting in two LANs - one the the modem and one to the other computer. As that computer is also connected to the Internet your computer has two access points to the Internet.
For a lot of apps this is no problem as it will tend to ignore one of them (it does not give you twice the Speed to the Internet).
iChat is different in that it, when in Video or Audio Only or Screen Sharing has to be able to tell you where it is in relation to the Internet in a very Linear way - as in Modem <-> any router <-> Computer and not
Modem1     modem 2
    \           /
Computer <-> Computer
The same issue occurs if you are Ethernet and Airport to your own Router or Modem as you will get two different IPs that way.
MIMO devices complicate the issue further in that they effectively split the WiFi band in to narrower portions allowing the computer to somewhat subdivide it's In and Out data. iChat "Sees" this as Multiple Internet Connections.
You could look at it like living in a Block of Flats/Apartments where the postal address is 35, Any Street when it needs to be Inner Dwelling 5, 35, Any Street
MIMO is then like iChat is trying to say "I am in rooms 1, 2, 3, 4 and 5 at inner Dwelling 5, 35, Any Street" which it knows it can not be.
This is a very simplified view but is essentially what is happening.
The upshot is that iChat has issues working with MIMO devices (And any other sort of dual hook to the Internet).
MIMO can rarely be turned Off and the software processes in the device that does it also bleeds over into the Ethernet connection.
I hope this helps.
11:51 AM Sunday; December 7, 2008