you do not need to be on the internet to use iChat.
You can use rendezvous.
If you have the AIM part activated and not on the internet with your network it will time out and tell you it could not connect.
As to using Bluetooth, If you are reffering to a Bluetooth network, I doubt it.
At first read I thought you were talking about using a Bluetooth headset with iChat but after re-reading it looks like you mean a bluetooth network.
You can do an Ad-Hoc computer to computer connection with Airport or connected to the same wifi network.