As Rohan said, you need at least a device with a wireless card that can be set on "monitor" or "promiscuous" mode, so you can sniff the packets sent between the clients and servers of the wireless network.
So, technically it is possible, but at the beginning it would be a very device-specific solution
technically no. however you can jailbreak it and install a OS on it and it would look just like a normal OS. just a smaller version. once thats done you can install softwares like aircrack exct. and it would capture the packets and then crack them.. now this does take some time.. anywhere from 15 minutes to a few days all depending on their encryptionsetings. but the average wifi shouldnt take more then 30 minutes.. tops. so yes in theory it is "possible" but it all depends on how much work you want to put into it in order to get it. there is no "app" YET.. im not sure if anyone is converting a software like aircrack from an OS to app compatable.. although im sure someone is somewhere.