I just knew that if the first you hear of something like this is from the mainstream press, then it's just a few less-than-bright friends of a journalist made the same setting change.
I saw a breathless, panicked post on some forum the other day where a guy thought he'd found a huge security flaw in iOS5- it seems if you double-click the home button and load the camera, and then press the home button, it goes straight to the home screen, bypassing the lock screen!
People variously could and could not replicate it, until someone finally mentioned that the lock screen only shows if it's been locked for more than a (user-configurable) length of time.
So when people who never use the lock screen went to turn on the lock screen and immediately test it out, shockingly they didn't have to enter a PIN!
While Siri is turned off by default, it appears that Siri-while-locked might be turned on by default once Siri is enabled. I wouldn't enable it in the first place if I had seen it, so it was news to me when I saw that it was turned on when I went to change my passcode the other day.
If it's enabled in that manner by default and lets you bypass the lock screen, then there's a serious problem.