Until August 2012, messages sent through the WhatsApp service were not encrypted in any way, everything was sent in plaintext. When using WhatsApp in a public WiFi network, anybody was able to sniff incoming and outgoing messages (including file transfers). The company claims that the latest version of the software will encrypt messages [but] their encryption is broken.
As I public service announcement, I hope you haven’t been sending anything private or confidential through Whatsapp. If you need to send something secure, consider using iMessage (for now, until proven otherwise), otherwise, consider using GPG encrypted email.