http://www.whatsapp.com/faq/#32

is it possible for someone to make an app like this? also, im guessing its totally impossible for the developer to make a version that would work with other phones besides iPhones right? this is one thing I will regret not getting the iPhone for, because lots of my friends are overseas with iPhones and an app such as this if I had an iPhone would make contacting them so easy. : /