I'm finding that if I do a hard reset on my iPhone 4, or the battery dies, and is recharged and the phone powered back up, Whatsapp will not open.
Messages are still coming through the app, but I cannot launch the app to read them. The app will attempt to launch, then shuts down. If I try 5 or so times, the app will simply delete itself from my phone. A few of my friends are also having this issue with the app on their iPhones as well.
Also, I'm not accidentally deleting the app. It simply disappears from my phone after a few failed launches.