OK, imagine this - with the rumors of new Apple TV OS that allow for third party apps. You could have Vjay mixer app which could accept more than one Airplay stream.
People could collaboratively mix visuals using iPads and Apple TV as output mix.
I know it's a wild idea and definitely nothing that serious pro would use (I bet hardware video mixer still rules) but it would be so much fun. Maybe even integrate it with Djay app so you could add music to the output via ATV.
I can see that for some quick gigs using iPad + vjay app and Apple TV plugged to projector would be OK if they add bit more features. It'd be like super portable VJ rig.