Probably working with carriers and hand set manufactures to try and get WhatsApp installed by default.
Where I live (in South America), carriers give WhatsApp traffic for free to their clients. I guess this is probably the case in other countries as well.
There's also the B2C communication side of things. Businesses would be willing to pay to communicate with their customers via WhatsApp. Disclaimer: Until this year I worked for a company building apps to let businesses communicate with their customers via SMS and Facebook. Support for WhatsApp was the next most asked-for platform from our customers and prospects.
They had him around in a bid to come up with one, but now he's leaving because they all agreed it was time to give up. ;)
The business model of start-ups like WhatsApp is centered around M&As, and here the result for WhatsApp has been spectacular.