It appears a WhatsApp client is coming to Mac OS X and Windows, according to information uncovered by software sleuths WABetaInfo. The information seems to come from a survey on WhatsApp's translation page and pretty clearly indicates a beta is on the way. Although the text is rough in parts, bits like "Download for Mac OS X", and "Download for Windows" make it difficult to imagine another scenario.
The chat software is used by a staggering 1 billion people and is already available on Windows mobile, BlackBerry, Nokia, and Symbian, so it would not be surprising in the least to see native Windows and Mac clients appear. Should it happen, expect sync functionality across different devices and operating systems.