To use Air Playit, do both the PC and iPhone need to be in WiFi connection or Bluetooth connection? Or just any internet connection?
iPhone only goes on 3G/4G/WiFi. PC usually has wired or wireless broadband connection.
According to the developers website, your phone does not need to be connected to Wi-Fi in order for this application to work. You can be on 3G and stream your music as long as your computer is connected to the internet.