If you've got an iPhone, pretty much everything you have done on it has been temporarily stored as a screenshot so that Apple can provide you with that cool zoom out effect when changing screens.
Zdziarski explained in a webcast that a screenshot of your most recent action on the phone is taken (ie sending a text message, e-mailing or browsing a web page) in order to cache it. The phone of course deletes this screenshot when you close the application; however, it can be retrieved.
Forensics experts have even used this security flaw to successfully catch criminals who have been accused of rape, murder and drug deals, according Zdziarski.
"There's no way to prevent it," Zdziarski said during the webcast. "I'm kind of divided on it. I hope Apple fixes it because it's a significant privacy leak, but at the same time it's been useful for investigating criminals."
In a side note, Zdziarski also demonstrated a rather lengthy workaround to the iPhone's passcode. So if you have anything top secret on your iPhone be aware that it can be retrieved no matter what protection you have in place!