So xbmc on my Pi does Airplay but it doesnt currently do mirroring. Reflections does do mirroring from a ipad to a pc, not seen Airplay yet.
Samsung tv's / phones and tablets all have allshare which can also be installed on to a laptop or pc for free. No need to buy another device like WDTV or apple tv. Anything on an allshare device is available on any other allshare device.
Not sure ref raspberry pi etc but I do know that there is plex app for ipad / iphone ( not sure ref android though )
Although requirements for plex on the windows platform requires more RAM etc
Download Plex for Windows
Long shot - was just on the off chance that it may be any good ?
Slightly hijacking the thread but how do ATV users get around the annoying fact that if you switch it off it needs the password entering next time you use it again. We have one we are trying out and the teacher who would probably best use it has his SmartBoard as his one and only screen - he doesn't use a connected monitor! Reconnecting the ATV in his classroom involves entering the wireless password and of course it goes in in plain text - so not doable in front of our lads!