I am not sure that spoofing an IP address would work for a chat session, because you need to get the responses from the person you are chatting with.
First, lets make sure we are all talking about the same thing. When I refer to spoofing an IP address, I mean that if your IP address is 1.2.3.4, you are telling the remote server a different IP address such as 2.3.4.5. Hiding your IP address in not the same thing. When you hide your IP, you are passing your traffic through a proxy so that the party you are chatting with can only see the IP address of the proxy server.
To give an example of spoofing, there is a method of denial of service attack called "smurfing" where you can flood a network with ping requests. When you launch the attack, you send a ping request to the network's broadcast address (so to knock out 192.168.1.100, you would send pings to 192.168.1.255) spoofing the address of the target system (192.168.1.100). This would cause every system on the network to respond to the ping, but send their responses to the wrong IP address.*
In chat, you do want to recieve the responses from the other party. A normal chat session goes as follows:
Code:
1.2.3.4: To 1.2.3.5|From: 1.2.3.4|Hello
1.2.3.5: To 1.2.3.4|From: 1.2.3.5|Hi there.
If you spoof your IP address, the person you are chatting with will not be able to respond, since their replies will go to the spoofed IP address:
Code:
1.2.3.4: To 1.2.3.5|From: 2.3.4.5|Hello
1.2.3.5: To 2.3.4.5|From: 1.2.3.5|Hi there.
2.3.4.5: To 1.2.3.5|From: 2.3.4.5|Uh, who are you?
* I used smurfing as an example since most modern networks disable or disregard traffic on the broadcast address, and as such the attack is no longer effective. Still, do not attempt.