Are there any chat clients or stand-alone clients that can interact with the iMessage server? I just switched from OSX 10.8.x to Ubuntu 12.10, and it's really the only thing I miss.
This message is quoted from http://apple.stackexchange.com/a/66047:
iMessage is based on a propietary, binary and encrypted protocol (source Wikipedia) with a client side device certificate, which makes it unlikely to be reverse engineered and implemented in third party apps any time soon (although attempts are being made).
Just like APNS it sets up a Keep-Alive connection with the Apple servers. Every connection has its own unique code, which acts as an identifier for the route that should be used to send a message to a specific device. The connection is encrypted with TLS using a client side certificate, that is requested by the device on the activation of iMessage.