A security flaw on the iPad 2 allows you to circumvent the passcode and access the last open app on Apple's tablet. The hack requires iOS 5, an iPad 2 Smart Cover and a certain sequence of events to perform.
To pull off the hack, install the Smart Cover on your locked iPad 2 then open the cover. From here, you will be presented with the passcode screen. Tap Cancel then hold down the power button on the top of the iPad 2 for a few seconds to activate the power down slider. But instead of powering down, close the Smart Cover, wait a second, then open the cover once again. Press Cancel to close the power down slider and you will be put back into the last application that you were running.
If you are put into the Home Screen, you can't actually access any apps but you can play media. The real risk comes if you were in Messages or Mail, as any sensitive data can be seen by anyone using the hack.
The flaw seemingly only affects iOS 5. I tested the hack on my iPad 2 running iOS 4.3 and was unable to replicate the issue, although once I upgraded to iOS 5, that all changed. 9to5Mac has discovered that you can temporarily fix this issue by disabling Smart Cover unlocking in the iPad 2 settings menu under the General tab.
Apple will likely address this issue as well as a security flaw for Siri in an upcoming software update.