It’s an idea of an italian startupper, Manuel Zanella: a SIM that works (in roaming) with all carriers and that permits only WhatsApp traffic.
The SIM permits only text traffic on whatsapp: no call, no SMS, no internet surfing.
To send multimedia contents through whatsapp you need to buy credits.
You can buy the SIM directly from the site, at a cost of EUR 5
Here the press release: http://www.whatsim.com/en/press/press-release-en
It will be a winning idea or will be a flop?