The Passcode Lock implementation in Apple iOS before 6 does not properly interact with the "Slide to Power Off" feature, which allows physically proximate attackers to see the most recently used third-party app by watching the device's screen.
https://exchange.xforce.ibmcloud.com/vulnerabilities/78683
http://support.apple.com/kb/HT5503
http://lists.apple.com/archives/security-announce/2012/Sep/msg00003.html