It is looking more and more like ArrowChat is going to require a sub-domain for at least the first version. If this is the case, only 1 sub-domain is required and it can already be in use by something else if you have a limited number of sub-domains.
This is because ArrowChat will keep a constant connection to the server to retrieve information instead of making queries every 3 seconds. Additionally, this will allow extremely large sites to have their sub-domain hosted on a completely different server, thus, having a server dedicated to chat.