As far as I know there are currently no messenger programs for the XBox save for sending messages within Kai and such.
The reason is that the XBox can not run more than one program at once.
For even if you ported something like GAIM, all you'd be able to do is message. You couldn't message with XBMC playing in the background or anything.
It has been suggested to the XBMC crew to integrate a messenger program into XBMC but they don't seem interested in it.
As far as I know, no one is working on such a project.