iChat doesn't have ongoing, active chat rooms. The chatting is intended for private communications between a group of colleagues or friends/family.
A server (let's say a spare Mac at a company) can create a chat room and leave it open permanently, but the only way to find it is to know the name of the "room," and even then the "admin" (the person who set up the room) has control of who gets in. Mac-Forums COULD have one if they wanted; ehMac (a site for Canadian Mac users) has one, but of course most of the time it's empty.
IRC has always been, imo, kind of a PITA as a method of chat: I was quite active on it in Doctor Who chats, but as the Internet got easier and forums such as this one sprung up I didn't find IRC and its myriad DOS-like commands to be so attractive anymore.