Install WhatsPad from Cydia. Then purchase whatsapp in iTunes. Then copy the ipa from iTunes to dropbox. Then open Ifile to copy the ipa to a local folder. Use something to install the ipa and voila.
it's a bit sad that even though I own a copy of whatsapp, I had to get the ipa from other means. Hopefully this makes whatsapp devs a bit more relaxed about just allowing ipads to run it out of the box.