Perhaps it's country-specific? I just double-checked and the App Store won't list it in the search on my iPad under iPad-only or iPhone-only, nor in Purchased. I'm in the UK.

When I first bought WhatsApp, I bought it in iTunes on my Mac. I only had iPod touches and iPads at the time, and it wouldn't sync the application to any of them. I seem to recall that I tried to follow a direct link to the App Store entry from Mobile Safari, and it said that my devices were incompatible.