How to update iPhone SDK without having to re-download Xcode?

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.

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.