When creating an iCloud account, you can use any Apple ID as long as it is a full email address and not a MobileMe account. If you have a MobileMe account, you must move that account to iCloud. You can find more information at: http://me.com/move

The NSNetService class and CFNetService APIs do not include P2P interfaces by default. To browse, register, or resolve services over P2P interfaces, an application needs to use the Bonjour DNSService*() APIs noted below. For more information, see Bonjour over Bluetooth on iOS 5 and Later.

Setting the interfaceIndex parameter to kDNSServiceInterfaceIndexAny in the following API's will not include P2P interfaces by default. To include P2P interfaces, you must now set the kDNSServiceFlagsIncludeP2P flag when using kDNSServiceInterfaceIndexAny or set the interfaceIndex to kDNSServiceInterfaceIndexP2P. The affected APIs are:

• DNSServiceBrowse

• DNSServiceRegister

• DNSServiceResolve

• DNSServiceRegisterRecord

• DNSServiceQueryRecord

iOS 5.1 introduces a new API to mark files or directories that should not be backed up. For NSURL objects, add the NSURLIsExcludedFromBackupKey attribute to prevent the corresponding file from being backed up. For CFURLRef objects, use the corresponding kCFURLIsExcludedFromBackupKey attribute.

Apps running on iOS 5.1 and later must use the newer attributes and not add the com.apple.MobileBackup extended attribute directly, as previously documented. The com.apple.MobileBackup extended attribute is deprecated and support for it may be removed in a future release.

Photos taken using iOS 5.1 can be deleted from Photo Stream on your device and will be removed automatically from Photo Stream on your other iOS 5.1 devices. Older photos can be manually deleted from your iOS 5.1 devices.

Provisioning profiles must be enabled for iCloud in the iOS Provisioning Portal. To enable a provisioning profile for iCloud, navigate to the App ID section of the iOS Provisioning Portal and configure your App ID for iCloud. After enabling the App ID for iCloud, regenerate your provisioning profiles to enable them for iCloud.

The setSortDescriptors: method of NSMetadataQuery is not supported.

In iOS 5, files that are protected via Data Protection cannot be used with iCloud Storage APIs.

File names are case-insensitive in OS X but case-sensitive in iOS. This can lead to problems when sharing files between the two using iCloud. You should take steps on iOS to avoid creating files whose names differ only by case.

Starting in iOS 5.0, in order to facilitate finer-grained playback control, a movie player is not automatically prepared to play upon creation. Call the prepareToPlay method to prepare the movie player. For more information, see MPMoviePlayerController Class Reference.

In iOS 5, signing a certificate with an MD5 signature is not supported. Please ensure that certificates use signature algorithms based on SHA1 or SHA2.

In 5.1 the UISplitViewController class adopts the sliding presentation style when presenting the left view (previously only seen in Mail). This style is used when presentation is initiated either by the existing bar button item provided by the delegate methods or by a swipe gesture within the right view. No additional API adoption is required to obtain this behavior, and all existing API, including that of the UIPopoverController instance provided by the delegate, will continue to work as before. If the gesture cannot be supported in your app, set the presentsWithGesture property of your split view controller to NO to disable the gesture. However, disabling the gesture is discouraged because its use preserves a consistent user experience across all applications.

This release of Xcode 4.3 is distributed as a single application bundle, [Xcode.app]. Delivering the Xcode tools in a single app bundle allows Xcode to be installed directly from the App Store, without the additional step of running the Install Xcode app. To develop for iOS 5.1, install Xcode from the App Store.

Within Xcode, you can launch additional developer tools, such as Instruments and FileMerge, via the menu item Xcode -> Open Developer Tool. You can then pin the tool on your Dock for access when Xcode is not running. Many of the tools previously included in the Xcode installer are now available as additional packages from the Downloads Preference Pane.