Screenshot behavior has been altered in iOS 7, as the release notes say:
Active touches are no longer canceled when the user takes a screenshot.This might seem like a minor change, and might seem innocuous enough, until one considers Snapchat's functionality.
As a security feature -- remember that Snapchat allows users to set a time limit for how long recipients can view a photos (up to 10 seconds), after which the images will "vanish" from the recipient's device. When a user opens a Snapchat photo, the photo is held open by a finger touch on the screen.
When one takes a screenshot, it touch is interrupted -- or was, at least, prior to iOS 7 -- which enables Snapchat to detect an "illegal" screenshot and notify the photo's sender that an image has been saved, even if the recipient attempts to keep a finger on the screen.
With the change, Snapchat images can be freely screenshotted and saved. Shh, don't tell anyone. Those who are using Snapchat to sex on iOS should find out if their "friends" have iOS 7 installed.
The good news for Snapchat is that iOS 7 is still months away, so the company has plenty of time to workaround the issue before iOS 7 launches.