Shareaza has chat features included, an instant messaging like feature to chat with other Gnutella clients (supporting it) and in the current dailys, we are trying out implemeting IRC chat in Shareaza...
But I don't think this is what you serach for because it is not what you want ("complitely decentralized") because IRC uses central servers for chatting...

Anyway, if you want to have a look at this you can get the latest dailys here:
Shareaza - Daily Builds


The problem with a really decentralised chat feature is to find back other chatting partners in the net. Which means there is a need to create something like an entire network only to be able to find your chat partners... And this increases enormeously the network traffic.
The other possibility is to use the current hash searching feature: Every client gets something like a completely random file with a unike hash, and other clients localise / find / search for clients by serching the hash of "their" file.
But like this, we have "de nouveau" the problem with the increased network traffic... and a second one because not all G1 clients support file search by hash...