Ok, so ive been using osx lion and ios5 since they launched.
ios5 is nice, notification center and imessage are fantastic.
airplay mirroring is an excellent idea too.
now, if osx lion had airplay mirroring it would be awesome, I wouldnt have to plug my macbook pro into the telly.
imessage would be cool too.
its not so important if its a separate app, or if its integrated into ichat or facetime.
if it is a new app then notification center is a must.