How do I detect when someone shakes an iPhone?

In 3.0, there's now an easier way - hook into the new motion events.

The main trick is that you need to have some UIView (not UIViewController) that you want as firstResponder to receive the shake event messages. Here's the code that you can use in any UIView to get shake events:

@implementation ShakingView

- (void)motionEnded:(UIEventSubtype)motion withEvent:(UIEvent *)event
{
    if ( event.subtype == UIEventSubtypeMotionShake )
    {
        // Put in code here to handle shake
    }

    if ( [super respondsToSelector:@selector(motionEnded:withEvent:)] )
        [super motionEnded:motion withEvent:event];
}

- (BOOL)canBecomeFirstResponder
{ return YES; }

@end

You can easily transform any UIView (even system views) into a view that can get the shake event simply by subclassing the view with only these methods (and then selecting this new type instead of the base type in IB, or using it when allocating a view).

In the view controller, you want to set this view to become first responder:

- (void) viewWillAppear:(BOOL)animated
{
    [shakeView becomeFirstResponder];
    [super viewWillAppear:animated];
}
- (void) viewWillDisappear:(BOOL)animated
{
    [shakeView resignFirstResponder];
    [super viewWillDisappear:animated];
}

Don't forget that if you have other views that become first responder from user actions (like a search bar or text entry field) you'll also need to restore the shaking view first responder status when the other view resigns!

This method works even if you set applicationSupportsShakeToEdit to NO.

From my Diceshaker application:

// Ensures the shake is strong enough on at least two axes before declaring it a shake.
// "Strong enough" means "greater than a client-supplied threshold" in G's.
static BOOL L0AccelerationIsShaking(UIAcceleration* last, UIAcceleration* current, double threshold) {
    double
        deltaX = fabs(last.x - current.x),
        deltaY = fabs(last.y - current.y),
        deltaZ = fabs(last.z - current.z);

    return
        (deltaX > threshold && deltaY > threshold) ||
        (deltaX > threshold && deltaZ > threshold) ||
        (deltaY > threshold && deltaZ > threshold);
}

@interface L0AppDelegate : NSObject <UIApplicationDelegate> {
    BOOL histeresisExcited;
    UIAcceleration* lastAcceleration;
}

@property(retain) UIAcceleration* lastAcceleration;

@end

@implementation L0AppDelegate

- (void)applicationDidFinishLaunching:(UIApplication *)application {
    [UIAccelerometer sharedAccelerometer].delegate = self;
}

- (void) accelerometer:(UIAccelerometer *)accelerometer didAccelerate:(UIAcceleration *)acceleration {

    if (self.lastAcceleration) {
        if (!histeresisExcited && L0AccelerationIsShaking(self.lastAcceleration, acceleration, 0.7)) {
            histeresisExcited = YES;

            /* SHAKE DETECTED. DO HERE WHAT YOU WANT. */

        } else if (histeresisExcited && !L0AccelerationIsShaking(self.lastAcceleration, acceleration, 0.2)) {
            histeresisExcited = NO;
        }
    }

    self.lastAcceleration = acceleration;
}

// and proper @synthesize and -dealloc boilerplate code

@end

The histeresis prevents the shake event from triggering multiple times until the user stops the shake.