This is a bit odd, but the highly anticipated WhatsApp is listed the Marketplace but it looks like you can't download it yet. The app will return an error saying "Can't Download: Sorry, You don't have permission to download this app".
Combine that with the app not showing up under new listings, the lack of description and images (there's one) and you know what guys? We're looking at an internal, private beta testing link. This is basically what happens when you try to download an app that you were not invited for to be evaluated--that's why it's not listed, there's no description, there is just one image, etc.
So our bet? This link (found here) won't be the final Marketplace link and we're still a few days from final release. So we're calling false alarm on this folks, sorry. Thanks, Maneesh V., for the link
Extra: For those who can side load, Giuseppe Spagnolo in WMPU comments, managed to rip the XAP from the Marketplace (there are legal programs that rip only free apps) and posted it here. You can get that here directly.