As far as I understand you cannot download the SDK separately - and even if you could, I wouldn't recommend it. Using the bundled SDK+Xcode release is the best idea in my opinion, as it will be working for you out of the box. I know it's annoying having to download such a large amount of data for an SDK (not all of us have 100mbps pipes to play with!) but for ease of development, it's the way I'd recommend you go.

For those who have installed Xcode but do not have the iOS SDK, here is the way to download it without re-downloading Xcode:Got to

Xcode > Preferences > Downloads

and select whatever you want to install.

Yes, contrary to what others say this is somewhat possible.

Method One

With this method you can download any iOS or OSX SDK and install it to XCode.Download the desired SDK, bundled with XCode, from developer.apple.com/downloads

Once the package is done downloading mount the disk image. Right on the XCode application in the image and click on Show Package Contents. Navigate to:

/Contents/Developer/Platforms/

the iOS SDKs are in:

iPhoneOS.platform/Developer/SDKs/

the OSX SDKs are in:

MacOSX.platform/Developer/SDKs/

and the iOS simulator SDKs are in:

iPhoneSimulator.platform/Developer/SDKs/

To install one of the SDKs simply find XCode.app and place the SDK in the appropriate folder of the XCode.app package.

Method Two

With this method you can only dowload iOS simulators and iOS simulator SDKs.

Open XCode and in the menu bar click on XCode > Preferences. In the preferences dialog select the downloads tab and select the simulators you would like to dowload.