Big Brother Camera Security is an intriguing free app that lets you know if someone is snooping around your iPhone. When you install the app you pick a passcode for your phone, and give it your email address. You'll get a confirming email with a code to validate it's you setting up the app, and once confirmation is complete you're ready to go.
Once the app has been launched, anyone who picks up your iPhone is asked to enter the passcode. If they fail to enter the correct passcode, the camera takes their picture using the front-facing camera. They get another try, and the app takes another photo. Big Brother Camera Security instantly emails you both the photo and the location of your phone.
It's a bit like Find my iPhone, but it provides a photo of who is currently playing with your phone, which I have to admit is pretty helpful. Of course, the app isn't just for locating a stolen iPhone, but if family members or coworkers are messing with your phone, it can help you catch them in the act.
Big Brother Camera Security had a checkered career with Apple. We covered some of the controversy here at TUAW. Because the author was analyzing passcodes to determine which were overused and thus unsafe, Apple objected and pulled the app. Apple also didn't like the app mimicking the lock screen. After a lot of submissions and design changes, Apple finally let the app back into the App Store.
I tried the app and it worked well after my first try delivered a distorted photo. I quit the app and didn't see the issue again. Digging deeper with the developer it appears the photos show up fine in Mail on iOS devices and in most third-party mail apps, but Apple has trouble displaying the images in OS X Mail. This issue will be fixed in an upcoming version of the software. Of course the app has to be running for all this to work, so you can't use your regular lock screen. Using the app locks your phone, but someone could reboot the phone and get around the app, so it is not as secure as the regular iPhone lock screen.