If you check goodereader, they should have a .bar file to sideload Whatsapp onto the PlayBook. There is some additional work involved in getting it set up, I think you have to use text message verification and then input the number from your phone to your PlayBook. Keep in mind that you will only be able to receive messages on one device. Also, unless you have a 3G/4G enabled PlayBook, it will only work when connected to WiFi. At least that's how it worked when I tried it on my PlayBook a few years ago. Not sure if it still works this way, but you can try.