How can I detect screen lock/unlock events on the iPhone?

May be you need to implement following methods in AppDelegate:

Tells the delegate that the application is now in the background.

- (void)applicationDidEnterBackground:(UIApplication *)application

Tells the delegate that the application has become active.

- (void)applicationDidBecomeActive:(UIApplication *)application

Tells the delegate that the application is about to become inactive.

- (void)applicationWillResignActive:(UIApplication *)application