If you are upgrading your iPad from iOS 4.2 GM Seed to iOS 4.2 GM, Xcode will extract symbols from the device the first time you connect the device. This process takes only a few minutes.

There is a new Base SDK setting called "Latest SDK". This is the recommended choice for all projects and will cause your project to always build against the newest available iOS SDK.

The Overview popup menu will display "Base SDK Missing" if a project was created with an SDK earlier than iOS SDK 4.2. However, after selecting a new SDK that Overview popup may still display "Base SDK Missing".

To work around the issue, close and reopen the project window.

The iPad screen rotation lock switch now functions as a sound/silent switch in iOS 4.2. This switch behaves the same way as the Ring/Silent switch on iPhone.

Applications targeted for iPad will need to take this into account when implementing audio behaviors.

For more information on audio behavior guidelines and how to implement them, see User Experience Guidelines in iPhone Human Interface Guidelines and the Audio Session Programming Guide.

Calendar can now import .ics files directly as a way to add events. If your app has access to .ics files, you should test importing them using the UIDocumentInteractionController API.

Applications can use the GKFriendRequestComposeViewController class to initiate friend requests.

The GameKit view controllers for leaderboards, achievements, matchmaking, and friend requests must be presented modally. Presenting them any other way will result in an exception and unexpected behavior. This affects GKLeaderboardViewController, GKAchievementViewController, GKMatchmakerViewController and GKFriendRequestComposeViewController.

In iOS 4.2, the local player is automatically authenticated again when an app enters the foreground. This will generate an authentication changed notification. If the app is linked with iOS 4.2 or later, the authentication completion handler will also be called (it is retained indefinitely).

During debugging, pausing or resuming an iOS application while it is backgrounded is currently unsupported and can result in undefined application behavior.

For applications built against iOS 4.2 and later, MapKit will now conditionally display annotation views based on the currently visible region of the map. To account for this, developers should make sure they reuse annotation views as documented and never assume that the absence of an annotation view implies the absence of a corresponding annotation

iOS 4.2 devices can print wirelessly only to printers that support AirPrint and are running the latest available firmware. Some currently available printers that support AirPrint are:

• HP Photosmart Premium Fax e-All-in-One Printer - C410

• HP Photosmart Premium e-All-in-One Printer series - C310

• HP Photosmart Plus e-All-in-One Printer series - B210

• HP ENVY 100 e-All-in-One Printer Series - D410

• HP Photosmart eStation Printer series - C510

Building an app that weak links CoreVideo.framework against the 4.2 Simulator SDK and then running that application against the 3.2 Simulator will result in a crash. The same configuration will work when building against the 4.2 device SDK and running on a 3.2 device.

The following frameworks currently do not support weak linking by way of the NS_CLASS_AVAILABLE macro:

• AV Foundation

• Core Animation

• Core Telephony

• MobileLaunchServices

When targeting older versions of iOS, you can check the class availability in these frameworks by calling the NSClassFromString function and seeing if it returns a non-nil value. For information about using the NS_CLASS_AVAILABLE macro, see iOS 4.2 in What's New in iOS.