At the moment (correct me if I'm wrong), the only streaming protocols XBMC supports are DLNA and Airplay (not mirroring).
Mirroring a device over DLNA doesn't make much sense, and Airplay Mirroring doesn't work on XBMC. Yet. I guess Koush could write an reciever for XBMC (which is what I think
he did for the Fire), but there isn't really much of an incentive for him to do so.
The most plausible scenario is that someone goes and writes an Airplay Mirroring receiver for XBMC. The reason it hasn't been done before, it that Airplay Mirroring (from Apple devices) is encrypted. Koush's Mirroring works because Apple TVs will happily accept unencrypted Airplay Mirroring streams; it's just that Apple devices
or the Apple Airplay Mirroring sources always encrypt their streams. But the details of the Airplay Mirroring protocol other than it's encryption are known, so writing an Airplay Mirroring sink is plausible. So, if someone did add Airplay Mirroring to XBMC it would work with Koush's mirroring, but not Apple devices. So, again, there isn't much of an incentive for anyone to actually go and write it.