How do I detect when someone shakes an iPhone?

First, Kendall's July 10th answer is spot-on.

Now ... I wanted to do something similar (in iPhone OS 3.0+), only in my case I wanted it app-wide so I could alert various parts of the app when a shake occurred. Here's what I ended up doing.

First, I subclassed UIWindow. This is easy peasy. Create a new class file with an interface such as MotionWindow : UIWindow (feel free to pick your own, natch). Add a method like so:

- (void)motionEnded:(UIEventSubtype)motion withEvent:(UIEvent *)event {
    if (event.type == UIEventTypeMotion && event.subtype == UIEventSubtypeMotionShake) {
        [[NSNotificationCenter defaultCenter] postNotificationName:@"DeviceShaken" object:self];
    }
}

Change @"DeviceShaken" to the notification name of your choice. Save the file.

Now, if you use a MainWindow.xib (stock Xcode template stuff), go in there and change the class of your Window object from UIWindow to MotionWindow or whatever you called it. Save the xib. If you set up UIWindow programmatically, use your new Window class there instead.

Now your app is using the specialized UIWindow class. Wherever you want to be told about a shake, sign up for them notifications! Like this:

[[NSNotificationCenter defaultCenter] addObserver:self
selector:@selector(deviceShaken) name:@"DeviceShaken" object:nil];

To remove yourself as an observer:

[[NSNotificationCenter defaultCenter] removeObserver:self];

I put mine in viewWillAppear: and viewWillDisappear: where View Controllers are concerned. Be sure your response to the shake event knows if it is "already in progress" or not. Otherwise, if the device is shaken twice in succession, you'll have a li'l traffic jam. This way you can ignore other notificat

You need to check the accelerometer via accelerometer:didAccelerate: method which is part of the UIAccelerometerDelegate protocol and check whether the values go over a threshold for the amount of movement needed for a shake.

There is decent sample code in the accelerometer:didAccelerate: method right at the bottom of AppController.m in the GLPaint example which is available on the iPhone developer site.